swapped the 11 inch discs in and now brakes are FIXED

read it as 'took them off' meaning unhooked everything. probably 'my bad'!

I didn't open anything up. no way is there air in the lines.

these are "new" ish calipers, from when I put in the 10.87's a few years back.

I will take them apart again and see if I put something together wrong. might run some sand paper over the rotors just incase.

Replaced the calipers in my B300 with rebuilts (one proper Mopar, one generic) about a year and a half ago, and when the temp falls below -20C (-4F) the generic leaks just sitting there overnight. Fluid runs out and down the tire. Looks like a small critter took a whiz under the tire in the snow.

Almost wonder if a caliper is hanging up?

Not sure if you have pin or slider type. But assume slider since that is what an M body would have. The slider type has been known to wear on the adapter and cause a hang up, how did the slider ways look?

no mention of new pads?..old pads with new rotors most likely did not burn to each other..

yeah, the slider type.
They seemed to move freely. I didn't notice any wear on the adapters, or the calipers that would stop them from moving.
it does feel like it has hit some limit though. no matter how much harder I press, it doesn't seem to be stopping any more.

nope, reused the old pads. Didn't really think about it. They really don't have many miles on them.

I did use new bearings though. lol.

(1) no air (2) only the L/R front corner assys were changed. (3) same pads (4) I'd brake kleen the pads/calipers (5 grab helper for the pedal (6) visual as he works the pedal (7) compare the old rotor(s) finish to the new rotor(s) finish (tho I dont think that if they are clean) (8) all that is left is a caliper piston travel problem from the retraction (mighta messed up the square seals) but both (L/R corners) appear to be fubared as it ain't pulling to one particular side. unless the retracting messed up both of em. Andrew holler how it turns out

so I pulled it out this morning. It no longer seems to be unable to stop.
it still has a bit of the old problem where it doesn't stop soon enough, or lock up.

I took off the master cylinder to see if the rod had backed down but it was still adjusted.
pedal has more movement without the master bolted up, so no binding there, nor is it the booster.

have never seen it, but can a master be bad and not move farther? or is it only the bleed by, which I have seen in the past?

I will start tearing into the brakes again to see if they are binding up somewhere.

is there a torque spec for the slider bolts? I just cranked down till they stopped.
never been a problem before, but just wanted to make sure.

took both sides apart. cleaned them again, took off some of the pad so it was "flat" again, incase it has molded to the old rotors.

with the pads off, and the sliders tighted down, I verfied I could move the calipers without any binding.

They are better, but about the same as the old set.

I think I have some other issue. If a master can give partial braking without moving further, or it could just be as simple as a bad booster again.

I cannot push hard enough to lock up the brakes, or get it to grab harder.
I literally was standing with both feet on the pedal and the car was still moving.
I think this is unrelated to the new rotor swap, as it did this before and I thought this might sort it out. but it didn't.

I would get a psi gauge and check your pressures at the calibers.
If your up around 1000+psi, then I would say glazed pads and rotors.

How old are your hoses?

anyone know shops that carry that kind of tester? I can't seem to find a parts house that carry's one.

The hoses are about 7 years old. maybe. replaced them with the original disk brake swap.

The reason I asked is the hoses can go bad internally and not show any signs of leaks.

web page

web page

I am at a loss. I doubt it is lines since it tracks straight.

I can be moving at 30 mph and stand on it, and it doesn't pull either way.

I have to believe it is the booster.

but since this is lack of power, it is harder to "prove" without that gauge.

And thanks for the web pages. I did find a few online. I was just hoping some national parts chain might carry them so I don't have to wait till next week to figure this out.

but it looks like an online order is at hand. I found one for about that same price with several adapters, so I might go that route.

well yesterday it was crazy bad.
today it is just regular bad.

when I had it inspected last year the guy mentioned that it didn't stop soon enough.
I just figured, heavy car, and all.
I had noticed when I first start out in the car sometimes, but just attributed it to poor vacuum build up after it had been sitting a while.
but I am guessing the booster is going.
wish there was an easier test than getting that test kit. it is about half the price of a new booster.
if I decide to keep going this route.

Two things you seem to be concerned about. 1) you believe the booster is interfering with travel.
2) you believe there may be a master cylinder issue.

To test #1, unbolt the master cylinder from the booster and step on the pedal to see if the pedal will travel all the way to the floor. If the pedal does not reach the floor, your issue is in the booster/pedal assembly. The vacuum assist of the booster is only an assist, meaning there should still be a mechanical connection and you should be able to overpower a defective booster with pressure on the brake pedal. Without a master bolted to the booster, the pedal should push to the floor fairly easily and smoothly. Get an assistant and measure the amount of travel the rod in the booster travels from pedal completely up, to pedal completely on the floor. If the pedal and booster operate correctly, reattach the master, and move on.
2) Now you have to determine if the master cylinder is the cause of your issue. With the master reattached to the booster. Loosen both brake line connections and see if the pedal will still go all the way to the floor. If it does, the issue probably in not in the master. It is possible the master plunger is not completely returning all the way to the back stop of the master cylinder, upon brake pedal release and it is not refilling with enough fluid to reactivate the brakes. If you tighten the lines you loosened for this test while the pedal is still down, you will not pull any air into the system. Once the lines are tight, you can remove the master from the booster to see if the plunger has returned fully to the back stop. I've seen corrosion build up inside the master that will not allow the plunger to return to the back stop and that can cause an intermittent poor brake function. Sometimes the rod between the booster and the master is too long. That will also cause the plunger to not return all the way to the back stop and cause poor brake conditions. If the rod is too short, it will not allow the master cylinder plunger to push enough fluid out of the master to reach full brake pressure (that could cause the pedal to stop before the pedal reaches the floor.) If you suspect the rod may be too long, you can add a couple washers on each stud between the booster and the master and see of that makes a difference in brake quality. If adding the washers does not make an improvement, remove them. The rod between the booster and the master is adjustable, adjust the length in or out 1/4 turn at a time (a little makes a huge difference). If the pedal reaches the floor with the brake lines on the master loosened, and the master's plunger is returning to the back stop, the master is not the problem, and we move on to the proportioning valve.

For a few years, the proportioning valve actually would block off fluid flow to one side of the valve or the other in the event of a line failure. Unfortunately, it would also do the same thing while bleeding the fluid if the bleeding process wasn't done easily. The process to recenter a shifted proportioning valve is not difficult, but requires a gentle touch. To test the valve, loosen either the fitting for the rear brakes, or the fitting for one of the front brakes, and step on the pedal and watch the fluid flow. Tighten the line, and loosen the other line (one line towards the front, and one line towards the rear) and step on the pedal and watch the fluid flow. If either the front or the rear line has little or no fluid flow, the proportioning valve has shifted towards the side with little on no fluid flow. To recenter the proportioning valve, you have to open the side with the fluid flow, and bleed off the fluid until you hear the pop or snap inside the valve, and immediately close off the line, and release pressure. Gently stepping on the pedal should leave the valve centered, and the brakes should be much better.

You have been messing with these brakes too long. Start at the beginning, and work through the process to this point and lets see what you have. Gene

poorboy thanks.
I had not thought about cracking lines open for the master cylinder check.

I did disconnect the master from the booster and verified full motion on the booster.
I am still concerned about lack of assist, but will check the master first now. But with the number of failed univeral boosters, I am not ruling it out just yet even if the master has some issue.

no proportioning valve on this car. was a single pot master, and all I did was run dual lines. (I know, I know, I should add one, just never got around to it).

I think you're running out of piston travel in the MC.
What kind of MC do you have now?

Check the booster by pulling off the vacuumhose, and see if the pedaleffort stays the same.

it is a 70's corvette master.
all the kits at the time only came with those.
I see some of the newer ones do mopar masters, but eh. power brakes is power brakes.

pedal effort is worse without vacuum, but pedal travel seems to be the same. I can't move it any further.

when I get a chance, I will try cracking a bleeder and see if it moves further or not.

I know you said no hydraulic lines were removed, but, to me, the system sure sounds airbound.

Here's and abbreviated procedure, as to how I confirm (or not) in minutes: Plug both master cyl outlet ports. Pedal should be rock hard (or close, with power booster). Not? Bleed master. (Can remain mounted, just slam on some c-shaped tubes).

OK now? Reconnect front fitting (to rear brakes). Still firm? Great. Not, trouble's out back, bleed, excess shoe travel, etc.)

Repeat with rear tube (for front brakes) connected.

The whole process can be done in a few minutes, and, if nothing else, localizes the problem area.

Rick

air in the lines to me would be a mushy pedal, or no pedal at all.
I have a rock hard pedal. I cannot push it past a certain point.
I have verified with the master removed from the booster, I can move it past that point.

now I have to see if opening a bleeder will let me move it past that point.

if not I guess bad master.
if it does, then I am still not sure where to look, but at least I know it isn't the 2 of those items lol.

I checked both calipers move freely. not sure where else it could be bound up. but I will get there once I eliminate the master.

Had the same problem with my old Toyota Landcruiser. Turned out to be the master cylinder seals (internal piston seals). Only some fluid was pushed down the lines as the brake fluid could circulate to some extend between the two chambers when the brake pedal was applied. New master cylinder solved the problem.

Stanton- I verified the adjustment was correct. you can look into the master to see that there is no piston movement in the master as I pushed it onto the booster.
as for fully retracted, I guess I didn't check that, but my brake light switch is a stop and it doesn't go out further than that. and my brake lights are not on, so it must be coming up to them.


DSG-I had wondered if there was another failure mode for a master cylinder. I have only experienced the fluid bleed by the piston, where you have no pedal. or it slowly falls.

Are you sure that the booster is good? Ive had a booster go bad and it was rock hard pedal. Bad master gave me really mushy pedal. Just because booster is new doesnt mean its good. On my moms Oldsmobile Cutlass Supreme went through about 6 bad masters right out the box before I got a good one. Similar deal once with an alternator as well.

no, I am not sure if the booster is good or not.
it is providing some assist, just not sure how much.
but I figured even if the booster wasn't working well. standing on the pedal, and I mean grabbing the steering wheel pulling myself out of the seat, both feet on the pedal would have locked it up, but it felt like I wasn't moving it any more at all.

so I want to check the master next by cracking a bleeder valve and see if it changes the pedal push distance. I will go from there to see what to do next.

I had the same issue, to a lesser essent, in a car of mine.
Turned out the MC just didn't have enough stroke for the pedalratio in the car. It just felt like the pedal itself was bottomout on something, while you know there should be some more stroke in it.
Swapped in a random MC and the problem pretty much went away. Only need to fine tune the MC's piston diameter for best brake-performance now.

Andrew we need closure!

lol, you are going to need to have some patients then. I will tinker with it again this weekend.
I am looking into the 70's corvette booster now too. see if it will bolt up to the aftermarket bracket.
kill 2 birds as it were.
better booster with a lifetime warranty from a national chain, instead of these flyby night chinese boosters.
depends on what the master cylinder test shows. don't just want to throw parts at this.

the rears never locked up.
and I was going to switch to disc in the back too. got the kit, but waiting on a sure grip before I rip the rear apart for the kit.

so I figured I could wait and see if I needed it after that.
plus I have one from a 68 sitting in the garage, but the mix of chevy master cylinder and such seemed to screw me up, so I would need new lines. it just kept snowballing, so I never got around to it.

The rears not locking up is irrelevant. Dissimilar brake systems require a proportioning valve as disc brakes need higher pressure to function than drum brakes.

That's why I used the 'pushing rope' analogy - your trying to move something that requires 1000 lbs. of force with only 1 lb of pressure. It simply won't work.

A Proportioning valve and proper Dual Circuit master cylinder will give your car brakes.

You could always get an adjustable proportioning valve. That way, you can adjust it now for your disc/drum combo, but then later when you go four-wheel disc, have the added luxury of being able to adjust your front/rear brake bias.

perhaps I don't understand what a proportioning valve is supposed to do.
are you saying bad front brakes are because I don't have one?

my understanding is that all it does is limit the force applied to the rear's over the fronts. AND it has that block off if I lose one circuit.
it doesn't bleed pressure over from one circuit to the other. Would defeat the purpose of having a dual circuit system.

No prop valve ... here's how I see it ...

You apply the brakes, the rear cylinders fill and subsequently the m/c won't travel any further. Meanwhile the fronts haven't got enough fluid in them to do squat.

http://stoptech.com/technical-support/technical-white-papers/proportioning-valves

this article says, it isn't doing that. it just helps prevent rear lockup's or over bias.

Just to add to the confusion, i had a car i put one of those booster/corvette master cylinder kits on, with a combo valve, and the brakes still sucked. Throwing the master/booster in the trash and installing good parts solved my problem. In my case, the master didn't have enough travel. Loosen the lines and see if the pedal will go to the floor. I'm betting it won't. Gene

If I understand the OP, there are no valves of any sort in his system, the front m/c line feeds the front brakes, the rear feeds the rear. The rear cylinders are smaller and fill before the fronts. With nowhere for more fluid to go, the master can no longer move and subsequently the fronts get no more pressure.

corvette 1974 master cylinder.
stock 10 inch drum rear's for a 65 coronet
stock mbody calipers up front.

Whats to say the pedal ratio is correct for the GM master cylinder? I know from building my own set-up years ago to put the master cylinder behind and under the floor that ratio was a huge driving factor in the performance.

I beiee some here are unsure how a dual circuit master cylinder works. They are under the impression that both pistons are connected to the pushrod. Only one piston is actually connected to the pushrod,the other is activated by hydraulic pressure and is free floating in the bore. The only time the pistons actually contact each other is when there is a failure of one circuit(pimar circuit, closest to the pushrod). This allowed the hydraulic cushion between pistons to fail and they then stack solid

says it is
1-1/8" Bore Dual Reservoir Master Cylinder

problem is this is a gm master cylinder. not sure on the interchange to get a 1 inch bore.
searches show some aftermarket stuff that might work. but I want to steer clear of that if I can.

I decided to pick up the corvette booster and master combo from autozone. So I don't have to go out again, if I find it is the problem.
if it isn't I will just return it instead.

I would really like to find a more "factory" solution but not sure it is out there with how cobbled together this is.

I did mention it the other day, just got around to playing with it last weekend.

If you have room for a Volare booster and master cylinder, I'm betting your brakes will be much better.

If you used a pedal assembly from the same company you got the booster/master from, your pedal ratio is no where near correct either. That company sells a lot of junk under several names. Someday it will catch up with them. Gene

so to the person that guessed no pedal left. you win.

I cracked open the bleeder screws and it still didn't go down any further than if they were closed.
I know the pedal and booster have more movement because I disconnected the master and I can put the pedal to the floor.

looking at the corvette booster and master combo, there is no way to make that work with this bracket setup. the rod on the back of the booster is too long. it would be engaging the booster when I bolted it up.

not sure if replace just the master is the answer. maybe as someone above said, internal bleed by so it isn't giving enough force, and that might solve it.

going to a smaller bore doesn't seem to be a good idea, if I am maxed out on distance already.

the volare one is too big in diameter.

I did find that the a body ones are 7 inch boosters and would bolt in, but not sure about braking force.

just finished. update in the original post.

Just my usual 2¢ worth...

The pressure at both ports on any non-stepped bore master cylinder should be virtually identical (assuming no external leakage), except for the instant of pedal application...more in a moment.

One reservoir is always larger on disc/drum masters. The large-reservoir end must be plumbed to the the disc "end" of the car (read: front). This is because discs have no adjusters, the piston slowly moves out as the pads wear, taking the fluid level down with it.

Under normal operation (again, no leaks), the secondary piston the the master "floats" and is applied (moved) via pressure created by the front end of the primary piston. This means that the primary side of the master will have full pressure a few milliseconds before the secondary. Plumbing the primary outlet to the fronts, as on Mopars, will tend to reduce poor-road-friction rear wheel lockup, and partially compensate for the extreme "grabbiness" (self-energizing servo action) of our Bendix rear drums.

Thanks for allowing my drivel...

Rick

Just another good reason to stay away from non Mopar parts. These conversions aren't hard at all, and work when factory Mopar parts are used.

while it is true "factory" mopar parts are way more expensive.
just finding a booster alone would have been more than this whole kit.
even after "fixing" it I am still up compared to a mopar swap.

it is funny, for 60's mustang's and 60's chevy's you can find a bendix dual diphram booster for around 100 bucks. for a mopar it is minimum 200 for a knock off. 350 for a real one IF you can find one.

even modern mopars, are about 50 dollars more than the equivalent non mopar modern booster.
I did a search on every booster autozone sells and you can see the crazy disparity.
even a volvo booster, which has to have fairly low volume was cheaper than the intrepid booster.

so yeah, would I have liked to use mopar parts. sure. but for double the money? not really that big a deal to me as long as it works.