Moparts
Brake pressure at caliper...700 psi?
Posted By: karls67gtx
Brake pressure at caliper...700 psi? - 11/21/10 04:01 AM
I have a question about front brake pressure. I recently purchased a SSBC pressure gauge to test my system and found only 700 psi at the caliper on both sides. I just put a new MC (15/16) from DoctorDiff. I think I should have more, like 1000-1200. I think this would solve my brake issue of never being able to lock the front brakes up. Could the rubber flex hoses be bad? I have an adj prop valve on the rear line. I still have the original combination block from the drum to drum setup. The calipers are 73 b-body stock discs. thanks for any help.
-karl
Posted By: autoxcuda
Re: Brake pressure at caliper...700 psi? - 11/21/10 04:14 AM
Quote:
I have a question about front brake pressure. I recently purchased a SSBC pressure gauge to test my system and found only 700 psi at the caliper on both sides. I just put a new MC (15/16) from DoctorDiff. I think I should have more, like 1000-1200. I think this would solve my brake issue of never being able to lock the front brakes up. Could the rubber flex hoses be bad? I have an adj prop valve on the rear line. I still have the original combination block from the drum to drum setup. The calipers are 73 b-body stock discs. thanks for any help.
-karl
What dia was your old master? What pressure was you old master?
How does it stop now with the 15/16" master?
Posted By: Challenger 1
Re: Brake pressure at caliper...700 psi? - 11/21/10 04:15 AM
First I think I would make sure I was using the stock proportioning/distrbution block for disc brakes.
Are you using the correct master cly for disc brakes?
Posted By: Twostick
Re: Brake pressure at caliper...700 psi? - 11/21/10 04:30 AM
When you say you have a prop valve on the rear line are you referring to the line coming out of the rear of the master cylinder or the one to the rear of the car? Dumb question probably but the line from the rear of the master is for the front brakes.
Kevin
Posted By: 70Cuda383
Re: Brake pressure at caliper...700 psi? - 11/21/10 07:13 AM
I always thought 700-800 was good for drum brakes, but 1000-1200 was idea for disc brakes.
Posted By: RapidRobert
Re: Brake pressure at caliper...700 psi? - 11/21/10 04:28 PM
I'd test the psi at the M/C itself to take the plumbing out of the equation then give the Doc himself a shout
Posted By: John_Kunkel
Re: Brake pressure at caliper...700 psi? - 11/21/10 10:45 PM
Pressure measured at the caliper will depend on the pedal ratio and how much foot pressure is applied to the pedal.
With a 15/16" MC and a 6-1 pedal ratio, 80 pounds of foot pressure will give about 700 psi at the caliper, if you want to see 1000-1200 at the caliper push harder on the pedal.
Posted By: karls67gtx
Re: Brake pressure at caliper...700 psi? - 11/26/10 08:39 PM
John,
I bled the brakes again and checked the pedal ratio, it is 6:1. After bleeding the brakes, I was able to achieve just under 1000 psi when pushing the pedal as hard as I could. I took it out and it I had to adjust the rear brake psi because it was locking up the rears like nothing. It stopped ok but the fronts still don't lock up. Is this normal? Should I change the stock calipers for some aftermarket ones? thanks for all the help...
-karl
Posted By: FuryUs
Re: Brake pressure at caliper...700 psi? - 11/26/10 08:44 PM
How does it stop now with the increased pressure and the backs not locking?
Posted By: karls67gtx
Re: Brake pressure at caliper...700 psi? - 11/26/10 08:49 PM
Not too bad, I just have to put my foot into it quite a bit. I just like to stop without all the effort...maybe I need to put power brakes....or upgrade the calipers.
Posted By: feets
Re: Brake pressure at caliper...700 psi? - 11/26/10 09:00 PM
Is the pedal hard to push? If so, try dropping one size on the master cylinder.
I made the mistake of putting 1-1/8" master on my brakes. At the time I was running the Dippy police brakes on the front and TSM kit out back. It took everything I had to make the pedal move and lock the fronts.
Since I switched to the big AMG brakes the same master cylinder feels perfect!
You've got small-ish brakes with a medium sized master cylinder. Drop down one size on the MC and it should do the trick.
Posted By: karls67gtx
Re: Brake pressure at caliper...700 psi? - 11/26/10 09:05 PM
I wouldn't say the pedal is hard to push, it goes down OK just have to push real hard to get a good stop. When you say drop one size, do you mean go up in size? I would love to but just purchased this new 15/16 from Dr Diff.
Posted By: dirtybee
Re: Brake pressure at caliper...700 psi? - 11/26/10 09:09 PM
i don't know about how much psi is needed but i put manual front disc for '73 up a-body on my '70 coronet (came with manual drum, drum). i put parts store master, rotors, calipers (the small ones), pads, and hoses and some repro front steel lines and didn't touch the brass prop valve/ tee and i don't really have to brake very hard, it stops like a new car. i think somethings not right if you have to stand on the brakes.
Posted By: feets
Re: Brake pressure at caliper...700 psi? - 11/26/10 09:18 PM
Is this on your 67 X? Was it an original manual 4 drum brake system?
I assume you used the slider style calipers. If the rear drums use the larger wheel cylinders the rears will be more likely to lock up before the fronts.
Your master cylinder is pushing the pressure out and the different sized front pistons and rear cylinders are causing you grief.
Posted By: Ledman_70
Re: Brake pressure at caliper...700 psi? - 11/26/10 09:40 PM
This may be useless info, but I recently installed a complete SSBC 11" front disc setup on my 64 Polara, including mc. I couldn't get the brakes to lock even on gravel, so I called SSBC's support line. I figured with 11" rotors and 4 piston calipers, the brakes should stand me on my nose, but the tech guy said they were designed like the original Kelsey-Hayes units Mopar used in the early 70's and were designed NOT to lock. I told him if I had known that I wouldn't have bought them. My buddy installed a 4-wheel Wilwood system on his 61 Falcon and Wilwood's tech support told him the same thing... his wouldn't lock either. I believe the thinking behind these designs is to make them anti-lock like new cars, but personally I don't like the control being taken out of my hands. My brakes stop the car ok, but not much better than the drums did, and the one time I got caught in the rain, they locked easily when I stepped on them with the same pressure I used normally. This sure seems like a poor design to me, but I'm not the best with brakes. I even bled mine with a power bleeder, but they're still mushy and I get NO air out during bleeding.
Posted By: karls67gtx
Re: Brake pressure at caliper...700 psi? - 11/26/10 09:52 PM
My 67 GTX was an original manual drum to drum car. I upgraded it when my front drums went bad. I was hoping for much improved braking but have not seen it yet...(10 yrs later). That is very odd about the SSBC kit, I would be very upset to spend that kind of money and not have it stop as good as a new car.
Posted By: feets
Re: Brake pressure at caliper...700 psi? - 11/27/10 05:05 AM
If the SSBC brakes are mushy and not air comes out of hte bleeders, it's time to pop the caliper off the bracket, hold the caliper at whatever funny angle you have to so the bleeder is the highest point, and open the bleeder until fluid comes out.
I had the same problem with my TSM kit. The bleeders looked to be at the highest point but a tiny bit of air was trapped above it. The funny angle gravity bleed fixed the problem.
As for the OP, was the size of the rear wheel cylinder considered when you ordered the MC?
Posted By: karls67gtx
Re: Brake pressure at caliper...700 psi? - 11/28/10 04:00 PM
So I did some more testing, I had my wife hit the brakes 3-4 times, pushing them down with two feet as hard as she could and I only saw 800 psi once at the caliper (avg'd 700psi). I need to go buy an adapter to check the PSI at the MC. I guess the question I have is, how do you achieve 1000-1200 psi? I found an article on Chevy Hgh perf (2009) that used a SSBC gauge like mine and had 1500 psi (power brakes)at the front caliper and 1400 out back. I am just wondering why it is such a big difference. The instructions with the gauge even say that manual discs should have 1000-1200. I have a 6:1 pedal ratio...just wondering where to go if I only have 700-800 at the MC? I did notice in the Chevy article that they installed the adj prop valve right at the MC, I have mine installed after the dist block on the rear line. thanks.
karl
Posted By: DoctorDiff
Re: Brake pressure at caliper...700 psi? - 11/28/10 04:13 PM
You could install a 7/8" bore master. That doesn't seem necessary because your brake system originally used a 1 1/32" bore master.
Posted By: karls67gtx
Re: Brake pressure at caliper...700 psi? - 11/28/10 04:57 PM
DoctorDiff,
Has anyone ever tested the MCs you sell to see what PSI they put out? I would think that most MCs with the same bore size(regardless of brand) would be very similar. thanks
-karl
Posted By: RapidRobert
Re: Brake pressure at caliper...700 psi? - 11/28/10 05:01 PM
I'm wondering if the prob is in the calipers/pads themselves or as said even something simple as a slight bit of trapped air, maybe just not enough to feel mushy
Posted By: FuryUs
Re: Brake pressure at caliper...700 psi? - 11/28/10 05:42 PM
I'm still wondering what the problem is exactly. Can't lock up the front brakes? What does that mean? The brakes aren't locked up from 0 psi until 1 psi less than locked up. So lockup minus 1 psi would be ideal, but since we don't know where that is, the system as it sits may be optimized. Or not. Is braking performance not up to standard? Is that because of the choice of pads? Rotor diameter? Are they not locking because the tires have more grip than the brakes? Rotational mass/inertia?
Posted By: DoctorDiff
Re: Brake pressure at caliper...700 psi? - 11/28/10 08:21 PM
I have never tested master cylinder pressures.
What is the bore size of your old master cylinder? How much pressure does it develop?
What size wheels/tires are you running?
What brake pads are you running?
Posted By: karls67gtx
Re: Brake pressure at caliper...700 psi? - 11/28/10 08:28 PM
Doctor Diff,
My old MC was 15/16 also but was leaky so I decided to upgrade to yours. I am running 15" cragars with BFG 235/60s up front with stock semi metallic pads.
FuryUs,
I wouldn't say that locking up the front brakes is the problem, more about inadequate braking. (about the same as drum before) hence the troubleshooting...discovered that brake psi should be 1000-1200, I only have 700, that feels like the problem, just not enough clamping force when brakes are applied, no matter how hard the pedal is pushed.
Posted By: feets
Re: Brake pressure at caliper...700 psi? - 11/29/10 04:34 AM
I think you need a smaller bore on the master cylinder to go with the new brakes.
Too large of a bore on the MC will make a very stiff pedal. You went from drum to disc so the fluid capacity of the brakes changed. That's where things went sideways on you.
Posted By: 70Cuda383
Re: Brake pressure at caliper...700 psi? - 11/29/10 05:45 AM
Quote:
I wouldn't say the pedal is hard to push, it goes down OK just have to push real hard to get a good stop. When you say drop one size, do you mean go up in size? I would love to but just purchased this new 15/16 from Dr Diff.
I know it seems counter-intuitive, but to increase line pressure, you go SMALLER on your master cylinder, or LARGER on your caliper piston.
it's a leverage thing. with a smaller diameter master cylinder, the same amount of effort you put into the system will move LESS volume, but create MORE pressure. vice versa, a larger diameter master cylinder will move a lot more fluid volume, but you won't be able to create as much pressure. it's like gears...shorter gears will not push you as far, but the leverage is much easier. taller gears will push you further per revolution, but the effort required is much higher.
so if you're having trouble generating the line pressures needed to get maximum braking effort, then drop a size or 2 in diameter of your master cylinder. if you have a 1", go to a 15/16" or a 7/8" if that's still too stiff, drop to a 3/4"
when I was converting my truck to manual brakes, I took the pedal ratio and size of the pistons in the front calipers, and calculated what size master I would need to generate 1200 psi. unfortunatly, I let the "expert" at Master Power tell me that I could never run a disc brake system on less than a 1" master, ever. so that's what I bought. my pedal effort is a little higher than I'd like it to be...but not enough to make me want to spend more money on the smaller master that I wanted.
Posted By: karls67gtx
Re: Brake pressure at caliper...700 psi? - 12/01/10 04:11 AM
Update on pressure testing, I was finally able to get to the store for an adapter to test the MC pressure....1000-1200...no problem. I am going to do some more testing but if I have 1000-1200 at the MC, shouldn't I have it at the calipers? Maybe I have some air trapped somewhere...thanks for all the help so far.
karl
Posted By: AndyF
Re: Brake pressure at caliper...700 psi? - 12/01/10 05:15 AM
Pressure at the caliper should be the same as at the MC unless you have a prop valve in the line. Trapped air or a kinked hose might also give you a lower reading. You certainly have some sort of issue with that brake system. Shouldn't be too hard to find the problem though. Just take it apart and measure the pressure at some different points and you'll figure it out.
Posted By: RapidRobert
Re: Brake pressure at caliper...700 psi? - 12/01/10 05:17 AM
Quote:
but if I have 1000-1200 at the MC, shouldn't I have it at the calipers?
yes. that only leaves a proportioning/crimped line/hose prob (somewhere intransit from M/C to caliper) or air or the calipers have a mechanical prob. Smaller bore M/C will give more psi (& you have plenty)
Posted By: karls67gtx
Re: Brake pressure at caliper...700 psi? - 12/02/10 03:00 AM
Pressure testing update...tested just before the rubber line on drivers side tonight, barely 700-800, which is what I am getting at both front calipers. I would like to just put a new T in for the front brakes to eliminate the distribution block. I can do this right? I have a rear adj prop valve to control pressure at the drums. I guess I am just wondering if this will be OK, don't really want to pony up the 139 for a new combo valve. thanks for all the HELP...almost there.
-karl
Posted By: RapidRobert
Re: Brake pressure at caliper...700 psi? - 12/02/10 03:18 AM
All I can add is I put 74 A discs/10" BBP drums/large bore M/C on a 65 Dart keeping the OE 4 wheel drum splitter & no other proportioning and it stops fine w no excessive wear on the front pads. Not the way it is supposed to be done but it's an around town slow speed daily driver & I've had NO issues.
Posted By: AndyF
Re: Brake pressure at caliper...700 psi? - 12/02/10 04:17 AM
Interesting. Maybe the brake switch has failed somehow and is blocking the distribution block? Or maybe the brake lines are full of crud? I'd keep testing until you find the problem.
Posted By: OLD318
Re: Brake pressure at caliper...700 psi? - 12/02/10 06:38 PM
Your low pressure at the calipers is due to 2 main factors: 1) MC bore and 2)metering valve
If you switch to a small bore MC 15/16"
you will have higher pressure but slightly further pedal travel
If you switch to a large bore you will have less pressure but a shorter pedal distance...
Ideally you want 1000psi at the calipers w/o having to leg the #### out of it!
When I switched my 70 Coronet to front disk from power drum, you have to switch the metering valve to a disc brake metering valve, that and a 1" bore MC with a disc brake booster (not power drum booster - I got the MP Brakes version) I have 1000 psi w/o legging it if I really put my leg into it it goes 1200+...
I tried the setup with the drum brake booster and metering valve, I couldn't get 1000psi with all the leg I could muster - max was 900psi the avg was around 750 psi... The brakes did not work very well. It stopped the car but it was not a
"significant improvement" over the 10" drums..
Once I got the right metering valve the pressure went up... But it wasn't until I added the disc brake booster that I got it where I wanted it..
I'm currently looking into a rear adjustable proportioning valve (w/ guage) to dial in the rears. When I really leg it (really hard sudden stop)the rear wheels will lock and skid...
I'd like to dial it down a tad in the rear and see if I can get it to just stop locking..
Oh well... yet another project...
Hope this helps - Best of luck to you!
Posted By: hooziewhatsit
Re: Brake pressure at caliper...700 psi? - 12/02/10 06:51 PM
Quote:
with a disc brake booster (not power drum booster - I got the MP Brakes version) I have 1000 psi w/o legging it if I really put my leg into it it goes 1200+...
I didn't think there was a difference between drum and disc boosters 
Is it possible your new one just applied more assistance? 
(I'm in the same boat; added disc all around on my truck, and while good, they're not great. But I don't want to hijack this thread)
Posted By: feets
Re: Brake pressure at caliper...700 psi? - 12/02/10 08:07 PM
I went from manual 4w drums to manual 4w discs and it made an IMPRESSIVE improvement.
Posted By: finadk
Re: Brake pressure at caliper...700 psi? - 12/02/10 09:48 PM
It is possible that the proportioning valve has "tripped" in other words when bleeding the brakes did it think there was a broken line to the rear shut of the pressure to the rears?
I think there is a way to reset it by really jamming on the brakes, you might want to do a google search about resetting a proportioning valve.
Good luck.scott
Posted By: jcc
Re: Brake pressure at caliper...700 psi? - 12/03/10 03:00 AM
Quote:
This may be useless info, but I recently installed a complete SSBC 11" front disc setup on my 64 Polara, including mc. I couldn't get the brakes to lock even on gravel, so I called SSBC's support line. I figured with 11" rotors and 4 piston calipers, the brakes should stand me on my nose, but the tech guy said they were designed like the original Kelsey-Hayes units Mopar used in the early 70's and were designed NOT to lock. I told him if I had known that I wouldn't have bought them. My buddy installed a 4-wheel Wilwood system on his 61 Falcon and Wilwood's tech support told him the same thing... his wouldn't lock either. I believe the thinking behind these designs is to make them anti-lock like new cars, but personally I don't like the control being taken out of my hands. My brakes stop the car ok, but not much better than the drums did, and the one time I got caught in the rain, they locked easily when I stepped on them with the same pressure I used normally. This sure seems like a poor design to me, but I'm not the best with brakes. I even bled mine with a power bleeder, but they're still mushy and I get NO air out during bleeding.
Say what? On a non abs system, the ONLY thing that PREVENTS brake lock up is your foot. Every brake system properly designed must be capable of locking up the brakes, since there are so many variables found when driving a car, weight, speed, road surface, trailers, etc
I am suprised it took so long for the downside of upping line pressure by decreasing MC diameter to be mentioned, increase in brake pedal travel, and if not careful with caliper piston sizes/numbers, lack of pedal volume. There are downsides to nearly every solution.
Did I miss anyone asking what the pad choice is, or if pads were corrrectly broken in and not glazed?
Posted By: Challenger 1
Re: Brake pressure at caliper...700 psi? - 12/03/10 03:17 AM
Quote:
I'm currently looking into a rear adjustable proportioning valve (w/ guage) to dial in the rears. When I really leg it (really hard sudden stop)the rear wheels will lock and skid...
I'd like to dial it down a tad in the rear and see if I can get it to just stop locking..
Oh well... yet another project...
Hope this helps - Best of luck to you!
Sorry to get off topic, OP. I already posted above to try to help.
Yes there is alot to gained with a rear brake proportionong valve. I have one plumbed into both my cars and can balance the front to rear and it does make a huge difference. 4 tires gripping the same is awesome.
I mounted the one on my 74 under the dash, I can adjust it while driving and it makes a huge difference in braking these old Mopars.
You can mine in this picture, it's the MP valve, took some creative tubing work. 
Posted By: Dakota_Don
Re: Brake pressure at caliper...700 psi? - 12/03/10 04:19 AM
try this, test the psi at the calipers,then remove the P valve on the front.. connect the front with a fitting to bypass the valve, then see what kinda PSI you have at the calipers.. that should tell you if the valve is bad, or whatever.. maybe it will narrow it down..if you have 1000 psi, with the valve removed then you found the problem,
Posted By: karls67gtx
Re: Brake pressure at caliper...700 psi?---SOLVED - 12/12/10 12:54 AM
Finally got some time to wrench on the brakes. I bought a simple T for the front brakes to bypass the distribution block. Put a couple of new fittings and bam...1000 psi at the calipers, 1100 on the second push. Drove it and could not believe the difference!
It stops with the same amount of effort in a new car with power brakes. I don't know why the old distro block would not push the 1000 psi but I will have to investigate further when the weather warms up. thanks for all the help!!
karl
Posted By: hooziewhatsit
Re: Brake pressure at caliper...700 psi?---SOLVED - 12/12/10 04:32 AM
Thanks for the update. Good to hear you (mostly) figured it out.
Once it gets warm again I'll have to find a pressure gauge and take a look at mine.
Posted By: Dakota_Don
Re: Brake pressure at caliper...700 psi?---SOLVED - 12/13/10 11:56 PM
kool
Posted By: Dakota_Don
Re: Brake pressure at caliper...700 psi?---SOLVED - 12/13/10 11:56 PM
what kinda gauge did you use??
Posted By: karls67gtx
Re: Brake pressure at caliper...700 psi?---SOLVED - 12/14/10 01:26 AM
the SSBC kit, I think I ordered it from Summit. works great!
-karl
Posted By: BDW
Re: Brake pressure at caliper...700 psi?---SOLVED - 12/14/10 01:54 AM
Good info, how did you measure the pressure at the caliper?
Replace the bleeder valve with the gauge?
Posted By: hooziewhatsit
Re: Brake pressure at caliper...700 psi?---SOLVED - 12/14/10 01:56 AM
Quote:
Good info, how did you measure the pressure at the caliper?
Replace the bleeder valve with the gauge?
If it was this gauge, it sure looks like you unscrew the bleeder and put the adapter in it. If you uncap the MC, you won't even have to bleed it again when you're done; just let it gravity bleed.
http://www.summitracing.com/parts/SSB-A1704/
I think I have something else on my wishlist 
Posted By: RapidRobert
Re: Brake pressure at caliper...700 psi?---SOLVED - 12/14/10 02:05 AM
since it's measuring psi rather than flow it only needs a miniscule opening (like a voltmeter only needs a very small gauge wire as opposed to an ammeter w large leads) so either the bleeder fitting or the hose whichever is easier to adapt a fitting to.
www.northerntools.com has a 0-1500 psi glycerin filled gauge (grade B accuracy) for $13.99 (1/4 NPT male). and a grade A accuracy similar type gauge (1/8 NPT male) for $17.99. All major credit cards accepted. Plus a fitting (or 2) to plumb it in
Posted By: BDW
Re: Brake pressure at caliper...700 psi?---SOLVED - 12/14/10 02:10 AM
Quote:
Quote:
Good info, how did you measure the pressure at the caliper?
Replace the bleeder valve with the gauge?
If it was this gauge, it sure looks like you unscrew the bleeder and put the adapter in it. If you uncap the MC, you won't even have to bleed it again when you're done; just let it gravity bleed.
http://www.summitracing.com/parts/SSB-A1704/
I think I have something else on my wishlist
That was what I was leading up to, if you remove the bleeder valve and replace with a gauge, will the reading be correct?
Or do you need to plumb in a T to the bleeder valve, and bleed before measuring?
Posted By: hooziewhatsit
Re: Brake pressure at caliper...700 psi?---SOLVED - 12/14/10 02:15 AM
Quote:
That was what I was leading up to, if you remove the bleeder valve and replace with a gauge, will the reading be correct?
Or do you need to plumb in a T to the bleeder valve, and bleed before measuring?
Looking at the picture of the adapters, I'd think you would need to screw the adapter in first, let it fill up with fluid, then screw the gauge on.
Of course the rest of the brake system before that would have to be bled out.
Posted By: RapidRobert
Re: Brake pressure at caliper...700 psi?---SOLVED - 12/14/10 02:29 AM
An air bubble(s) will absorb psi as it collapses but I'd think it'd peak & you'd get a correct reading for a second (just like brakes will pump up briefly) but they need to be bled anyhow so I'd do that as said then take a reading.