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Brake pressure at caliper...700 psi? #859006
11/21/10 12:01 AM
11/21/10 12:01 AM
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Smith Mtn Lake, VA
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karls67gtx Offline OP
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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


Mopar Man
Re: Brake pressure at caliper...700 psi? [Re: karls67gtx] #859007
11/21/10 12:14 AM
11/21/10 12:14 AM
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So Cal
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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?

Re: Brake pressure at caliper...700 psi? [Re: karls67gtx] #859008
11/21/10 12:15 AM
11/21/10 12:15 AM
Joined: Feb 2005
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Cincinnati, Ohio
Challenger 1 Offline
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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?

Re: Brake pressure at caliper...700 psi? [Re: Challenger 1] #859009
11/21/10 12:30 AM
11/21/10 12:30 AM
Joined: May 2003
Posts: 6,545
Downtown Roebuck Ont
Twostick Offline
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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

Re: Brake pressure at caliper...700 psi? [Re: Twostick] #859010
11/21/10 03:13 AM
11/21/10 03:13 AM
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Marysville, O-H-I-O
70Cuda383 Offline
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I always thought 700-800 was good for drum brakes, but 1000-1200 was idea for disc brakes.


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Re: Brake pressure at caliper...700 psi? [Re: 70Cuda383] #859011
11/21/10 12:28 PM
11/21/10 12:28 PM
Joined: Nov 2003
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Lincoln Nebraska
RapidRobert Offline
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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


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Re: Brake pressure at caliper...700 psi? [Re: karls67gtx] #859012
11/21/10 06:45 PM
11/21/10 06:45 PM
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Rio Linda, CA
John_Kunkel Offline
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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.


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Re: Brake pressure at caliper...700 psi? [Re: John_Kunkel] #859013
11/26/10 04:39 PM
11/26/10 04:39 PM
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Smith Mtn Lake, VA
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karls67gtx Offline OP
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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


Mopar Man
Re: Brake pressure at caliper...700 psi? [Re: karls67gtx] #859014
11/26/10 04:44 PM
11/26/10 04:44 PM
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Montana
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FuryUs Offline
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How does it stop now with the increased pressure and the backs not locking?

Re: Brake pressure at caliper...700 psi? [Re: FuryUs] #859015
11/26/10 04:49 PM
11/26/10 04:49 PM
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Smith Mtn Lake, VA
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karls67gtx Offline OP
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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.


Mopar Man
Re: Brake pressure at caliper...700 psi? [Re: karls67gtx] #859016
11/26/10 05:00 PM
11/26/10 05:00 PM
Joined: Jan 2003
Posts: 28,065
Irving, TX
feets Offline
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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.


We are brothers and sisters doing time on the planet for better or worse. I'll take the better, if you don't mind.
- Stu Harmon
Re: Brake pressure at caliper...700 psi? [Re: feets] #859017
11/26/10 05:05 PM
11/26/10 05:05 PM
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Smith Mtn Lake, VA
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karls67gtx Offline OP
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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.


Mopar Man
Re: Brake pressure at caliper...700 psi? [Re: karls67gtx] #859018
11/26/10 05:09 PM
11/26/10 05:09 PM
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Toronto, Canada
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dirtybee Offline
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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.

Re: Brake pressure at caliper...700 psi? [Re: karls67gtx] #859019
11/26/10 05:18 PM
11/26/10 05:18 PM
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Irving, TX
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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.


We are brothers and sisters doing time on the planet for better or worse. I'll take the better, if you don't mind.
- Stu Harmon
Re: Brake pressure at caliper...700 psi? [Re: feets] #859020
11/26/10 05:40 PM
11/26/10 05:40 PM
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Central Ohio
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Ledman_70 Offline
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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.


Jeff Adams 64 426 Polara
Re: Brake pressure at caliper...700 psi? [Re: Ledman_70] #859021
11/26/10 05:52 PM
11/26/10 05:52 PM
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Smith Mtn Lake, VA
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karls67gtx Offline OP
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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.


Mopar Man
Re: Brake pressure at caliper...700 psi? [Re: Ledman_70] #859022
11/27/10 01:05 AM
11/27/10 01:05 AM
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Irving, TX
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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?


We are brothers and sisters doing time on the planet for better or worse. I'll take the better, if you don't mind.
- Stu Harmon
Re: Brake pressure at caliper...700 psi? [Re: feets] #859023
11/28/10 12:00 PM
11/28/10 12:00 PM
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Smith Mtn Lake, VA
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karls67gtx Offline OP
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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


Mopar Man
Re: Brake pressure at caliper...700 psi? [Re: karls67gtx] #859024
11/28/10 12:13 PM
11/28/10 12:13 PM
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Polson, MT
DoctorDiff Offline
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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.

Re: Brake pressure at caliper...700 psi? [Re: DoctorDiff] #859025
11/28/10 12:57 PM
11/28/10 12:57 PM
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Smith Mtn Lake, VA
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karls67gtx Offline OP
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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


Mopar Man
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