1993 rear brakes not working

[laybackman]

1993 Mustang, 5.0, standard shift, bone stock unit.
Rear brakes are not working. New pads, drums, fluid, bled properly and adjusted properly trust me. The brake booster is not leaking and the check valve and manifold vacuum is good.
No brake pedal fade at all. Brakes do not stop the drums even with the wheels off, just the drums on and secured with lugs.
Backed off the brakes a bit to remove the drums, fired up the engine, just touch the brakes and the wheel cylinders do move outward.
Now the general opinion is that the proportioning valve is the culprit. BUT the proportioning valve is designed to allow MORE fluid to the rear brakes say if you break a front brake line, or if you do a panic stop. The proportioning valve allows extra fluid to flow back to the rear brakes.
My gut tells me that the master cylinder rear reservoir for the rear brakes is bypassing. It sends just enough pressure back to the rear brakes to move the unimpeded rear wheel cylinders BUT won’t move the wheel cylinder pistons once the brake shoes/drums are on.
Easiest way to test my theory?

[laybackman]

OK I have a scenario for you: Let us say the rear master cylinder reservoir piston is allowing fluid to by pass some, BUT the front reservoir is fine so the front brakes work. Would we end up with a firm pedal YET no real pressure for the rear brakes?

[65comet]

You open a rear brake bleeder and fluid squirts out when you press on the pedal? I mention that though you say it was bled, if you used a bleeder of some type it won't show how things are operating. If you get a good fluid squirt, then it is probably the wheel brake cylinders. No squirt, then work your way backward trying to break apart each line connection and press the pedal at each to see if there is a clogged line all the way to the proportioning valve. Then the lines out of then into it. From a snow area, road salt will rust the outside to cause breaks. I have seen some as they passed through narrow passages have the rust expand internally and crush the rust thinned line in onto itself and totally stop it up.

[laybackman]

Originally Posted by 65comet

You open a rear brake bleeder and fluid squirts out when you press on the pedal? I mention that though you say it was bled, if you used a bleeder of some type it won't show how things are operating. If you get a good fluid squirt, then it is probably the wheel brake cylinders. No squirt, then work your way backward trying to break apart each line connection and press the pedal at each to see if there is a clogged line all the way to the proportioning valve. Then the lines out of then into it. From a snow area, road salt will rust the outside to cause breaks. I have seen some as they passed through narrow passages have the rust expand internally and crush the rust thinned line in onto itself and totally stop it up.

This is essentially a one owner garage kept '93. It show little in the way of rust. We thought of this but I don't think this is the problem.
Brakes were hand bled BUT not done forcefully to see if the brake fluid would squirt. The brake fluid started out as a bit tainted but cleared up rather quickly. It is one clean little stock fox body trust me.

The brake pedal is not soft at all and there is no brake fade after sustained hard pressure on the brakes. The brake booster works. Press hard on the brakes until max hard deal is reached, fire her up and the pedal drops just a bit like it should.

What we did last was to see if the wheel cylinders moved the brake shoes with the drums off. I just touched the brakes and the wheel cylinders pushed the brake shoes apart.

Throw the drums on and their lugs then adjust the brakes and start her up. Put it in gear and hit the brakes and the drums turn unimpeded.

What we did NOT do when we first got movement of the wheel cylinders was, we did not see if we could force the wheel cylinders pintels back in by applying pressure to the brake shoes.

My thoughts are if that was possible then the rear chamber of the master cylinder is failing. No external leaks....wheel cylinders will move BUT Only when they are under little to no resistance. Once you add resistance in the form of drums the rear chamber of the MC bypasses.

Is it possible that the proportioning valve is NG?

We also thought about this: We 'bleed' one front brake while the car is running and the brakes gets applied. That would cause a false reading of a broken front brake line, maximizing brake fluid pressure to the rear brakes as it was designed to do. That may happen this weekend BUT My Son is contemplating replacing both the MC and the PV....'cause he is pissed!

[65comet]

When you are adjusting, do you do it till they are tight and the wheel doesn't want to move easily then take it back a couple of clicks till it just starts to move freely? I only mention because I've seen some people adjust them looser a few clicks after putting it back together without spinning the wheel. They know the adjusters are suppose to go in one direction but swapped them from one side of the car to the other and are thus loosening instead of tightening. Only mention because the best of us have had D'oh moments.

[laybackman]

Originally Posted by 65comet

When you are adjusting, do you do it till they are tight and the wheel doesn't want to move easily then take it back a couple of clicks till it just starts to move freely? I only mention because I've seen some people adjust them looser a few clicks after putting it back together without spinning the wheel. They know the adjusters are suppose to go in one direction but swapped them from one side of the car to the other and are thus loosening instead of tightening. Only mention because the best of us have had D'oh moments.

Good though but I adjust shoe brakes with the tires on the axle, so I know when those brakes have locked. Then I count the 'clicks' just until that tire will move with just a bit of scrape-noscrape-scrape sound.

BTW the BEST brake hoe setup I ever had was on an early MOPAR. They were adjusted using a ratchet or box/open end wrench since each shoe got adjusted turning a bolt head individually. Theoretically you could mix and match old/new brake shoes since there were no self adjusters. Took longer though and you really had to get the 'back off' gap right!

[65comet]

I wish I could give you more advice. It is probably something that if I was there would jump right out at me, but diagnosing over the internet isn't always easy. It sounds like you have the know how to figure it out this weekend. So I'm just going to say good luck.

[laybackman]

Originally Posted by 65comet

I wish I could give you more advice. It is probably something that if I was there would jump right out at me, but diagnosing over the internet isn't always easy. It sounds like you have the know how to figure it out this weekend. So I'm just going to say good luck.

We'll get to the bottom of it given enough time. My Son's schedule has been pretty full so maybe this weekend we can spend some time on this brake problem and get it doped out....hopefully.

I will post our findings IF we have any.

[OldSVTGuy]

When you push the brake pedal while it is running, does it slowly go to the floor? I had this and the MC was the culprit.


Usually, the bias is for one front brake and one rear brake to work if you get a line break.


You have a soft line above the rear end, correct? You might check if this is bad. Or, check for a line crimp somewhere on the rear end or from the front to the rear.



I've had lots of brake experience but nothing quite like this. My first Mustang was a 92 LX with rear drums. Never experienced what you are going through.


Good luck.