72455

One of the things on my "to do" list was to replace the master cylinder on my 72 Supreme. (It had a seal leak where it bolts up to the booster). I've been driving it with this issue since I bought it with no problems. I would just periodically check the fluid.

So today I thought I would take care of it. With my stepson's help, we got to it. Made the swap with no issues as far as the fittings being siezed onto the master cylinder.

Got the new one on, topped the fluid off and commenced to bleeding. After we got that done, started it up and checked the pedal...no brakes! (We had it up on jack stands). The pedal just goes to the floor and won't come back up.
To say I am frustrated is an understatement. (I am wondering if I should have left well enough alone)

After doing some research, it would seem that the proportioning valve is a poor design and can "lock up" after doing a bleed. I guess it's a failsafe design in case you lose front or rear brakes you still have the other.

So I made a run to Napa to scoop up a new valve, and when we tried to break the lines loose from the old one, they wouldn't budge. My stepson said the fittings and the valve are now one...nice.

His theory is that if we vacuum bleed the lines and release all pressure, the valve will free up and I should be good. I hope so.

Has anyone come across this issue or heard of the proportioning valve being problematic? I hate the thought of my Cutlass being down for the count and would welcome any feedback.

Thanks for the read, and wish us luck!

Dave
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oldcutlass

Did you bench bleed the master cylinder and the brakes after you installed it? You should still have one side or the other for brakes if the valve shifted over.

72455

We bled the brakes but didn't bench bleed the master cylinder. Could that be the issue? Usually if a master cylinder needs bench bled it will come with a kit, right? Also, is it too late to bench bleed the master cylinder now that we've installed it and bled the brakes?

Not sure if I have any brakes or not. All I know is I have no pedal and I wasn't going to chance it on a "test run."

Hairy Olds

Sure you can still bench bleed the master and I would. Find a kit at a parts store and bleed it.

72455

My stepson was one step ahead...he's doing it now 😀

joe_padavano

No, it's cast iron and water in the brake fluid can cause rust, which can cause the internal piston to get stuck. This isn't the fault of the design. Of course, not bench bleeding the M/C is a bigger problem.

oldcutlass

Unfortunately its hard to bleed it on the car because it mounted on an angle which could trap an air pocket. Its not hard to take back off, bleed, reinstall and re-bleed the brakes again.

72455

When we bled the rear brakes I would get no pedal movement when he cracked the bleeder, but good pedal movement when we bled the front. He said that it was more or less a trickle from the rear instead of a steady stream.

Allan R

Try using some heat on the connections to the proportioning valve. And when you try to undo the nut, be sure to use a flare wrench so you don't round it off.

The MC must be bench bled horizontal. It can take up to 10 minutes to get all the air out. I think that's the majority of your problem.

oldcutlass

Is the brake light on in the dash? Have you replaced the rubber brake hose that goes from the differential to the body?

72455

So would air in the MC cause the pedal condition at the front and rear I described above? He's already bench bled the MC, so should we install it and try again before we vacuum bleed the system?

72455

No light on dash and I should add that all was good until we swapped out the MC. Also, when I first bought the car, the rear chamber of the MC was empty (the drag racing life) so we topped it off, bled them down and as I said, all was good. So would you still suspect the hose?

Allan R

Air can cause a lot of issues in the brake lines to the rear and the rear wheel cylinders. If it's now bench bled, then you should be good to install and bleed the brakes. RR first, LR, FR and finally FL. If your brakes were working ok prior to installing the new MC I would bet your problem lies with the air not being bled out of the system.

Eric is referring to the center brake hose at the back of the car. It feeds the connection to the rear brakes. Sometimes those get rotten with age. They're a real PITA to replace too.

oldcutlass

It possibly can be the hose. Normally when the valve shifts it closes the switch attached and turns the light on at the dash to notify you of a brake problem. Is there a wire connected to the valve?

72455

I do remember seeing a wire, but in regards to the brake light, it didn't come on today or when my back chamber was empty from before, so maybe the bulb is out?

72455

I'm ever hopeful that bench bleeding the MC was the issue. My stepson is going to get back at it tomorrow, so I'll post updates as I get them.

Fun71

That "seal leak" was the master cylinder leaking fluid, which then leaked down the booster. I'll bet there is a puddle of fluid inside the booster, too. I had that happen to my car soon after I bought it in 1990. I siphoned the fluid out of the booster, then the booster failed a while later. Not sure if that was due to the fluid or the fact that every rubber part on the car was dried out from age and exposure.


All master cylinders need to be bench bled, but to save money these days the new master may not come with a bleeder kit.

Are you sure someone drained the rear reservoir for a cheap man's line lock? It would be really scary to try and stop that car at the end of a quarter mile run with only front brakes. Perhaps the fluid had completely drained out due to the faulty master cylinder?

truckman5000

Have to bleed it alot

72455

That may be so, but since that time when we noticed the back chamber was empty and filled it, it's been fine up until today when we swapped the MC.

72455

Trust me, we did. I bet we pumped and cracked each corner 20 times if we did it once...

Fun71

Ah, I mis-read your post about periodically checking the fluid. I was multi tasking while reading and when I was typing I thought you periodically added fluid.
So yeah, sounds as if it just recently started leaking. If the master cylinder is still off, check to see if there's any fluid in the booster. Hopefully not.

72455

Where would we check and how?

Fun71

I had the master off and put a rubber vacuum hose into the hole in the booster. It came out with the bottom 1" or so wet, so I syphoned as much fluid out as I could.

72455

Are you talking about where the plunger is or inside the booster? And what hole?

Fun71

Well, it was in the 90s so my memory isn't exact. May have been the vacuum check valve hole.

sysmg

In my experience it turned out the booster was bad and leaking vacuum. That pulled fluid past the rear master seal and out of the rear reservoir. I would recommend checking and possibly rebuilding your booster too. I think I went through 2 masters, before I realized this.

joe_padavano

Two things here. First, the only way fluid comes out the back of the master cylinder is if the seals in the M/C are bad. These seals are designed to withstand a couple thousand PSI when you apply the brakes. 14 psi vacuum from the booster is NOT going to pull fluid past a good seal. Second, if there is a vacuum leak between the booster and the M/C, the only thing that's bad is the seal between the two. You can replace this without replacing the entire booster.

bry593

Note that the proportioning valve has a button on one end to move the internal piston back to neutral. When you bleed the brakes, you are supposed to depress this button. At least that is how it works on my '70 Chevy.

joe_padavano

Sorry, but that's not how it works. Here's a cross section of the combo valve. Note the push button on the left. All that pushbutton does is disable the metering valve function. It does not have enough travel to even reach the piston, and you'd have to completely push the metering valve into the piston to do anything. More to the point, if the rear brakes are blocked, the piston is ALREADY all the way to the right. Pushing on the button isn't going to move it to the left.

oddball

FWIW, I've had several reman master cylinders bad out of the box.
Remove the big C clip on the back and pop out the innards and give it a good inspection. Make sure the bore is smooth and the seals are pliable and not ripped. You can also get rebuilt kits on rockauto pretty easily (if memory serves......). As long as it hasn't had water, a reseal will fix it up.
When I've had no pedal return it's been the master.

Allan R

^^^^ I can relate to that. My 'brand new' MC was bench bled, installed and brake system bled. It was fine till the next spring when, after having been in storage all winter and 0 miles it leaked out the back end too!

72455

Ok, so here's the latest update. My stepson was able to get me back on the road, but I still have no fluid going to the rear, so I'm just on front brakes right now. Could the diverter piston in the prop valve be causing this? Maybe it's stuck in the position of not allowing fluid to get to the rear, and if so how can I get it to move enough to move fluid so it will?

I'm also wondering this (and I know it's a stretch)...
As I have previously mentioned, there was no fluid in the rear chamber of the MC when I bought it. ( I drove it this way for about a month before I realized this)We added fluid and bled the rear, and all was good. So....what if I re create that situation? What if I drain the rear chamber, drive it, fill the fluid and bleed the rear? Maybe the air moving through the rear lines will shift the piston enough? Like I said, I know it's a stretch, but what have I got to lose?

oldcutlass

Did you check that the wire on the metering valve was connected? Is the brake light on at the dash? If not, does it light when you engage the e-brake? If the light works as described, then break the connection between then rubber hose at the rear end and the main steel brake line on the body to see if you have fluid there.

72455

See post #15

oldcutlass

I read post 15, that's why I asked again. If the wire is connected and the brake light comes on with the emergency brake, it almost rules out that the shuttle valve inside the metering valve has shifted. So if the light works with the e-brake and the wire is connected, remove the wire and ground it. The light on the dash should light. If it does the problem is probably not the valve.

don71

I think I'm following along here...so you're not getting much fluid from the rear bleeder screws..correct? Then why don't you try to crack open the line at the wheel cylinder it self and see if you can get flow. I didn't see anyone bring up wheel cylinders and their condition yet.

I would check that next, then consider the brake hose on the diff housing or the master still being bad even if new.

72455

I checked the light with the parking brake engaged this morning...light came on, so that rules out a bad bulb. I will try to find time this afternoon to ground the wire. Is that a different circuit than the parking brake light? And, if the light comes on, then do I move to the hose in the rear? Also, I took it over to my buddy's house this morning. He's going to check the adjustment on the rear shoes and do a pneumatic bleed on the system. I'll post an update later today.

72455

If the wheel cylinders were bad, then why would I not have had the issue before I changed the MC? Also, wouldn't my brake light be on? I do know that I don't have any leaks from the wheel cylinders, so I dunno what else would be the issue with them.

oldcutlass

Let us know what you find.

72455

Pneumatic bleed at all 4 corners, rear brake adjustment was good...no change. And the hell of it is, after all this, I still have the leak😕. Maybe I should put the old MC back on?? I'm frustrated...