[Wow what a write up, should be a sticky with this much step by step practical advise. I really appriciate your excellent advice and easy to understand directions. I am a paint and body guy so this kind of problem I do not have a good base of knowledge.
Thanks again and I will let you know what I find.
Rodney from SW Florida
quote=basstrix;260465]
Before you go spend $ on a m/c, I would verify the brake system is installed and operational.
Your car should not have a proportioning valve. On a drum brake car, that's just a distribution manifold with the pressure differential switch. Disc brake cars will have an additional valve called a metering valve. This metering valve requires additional internal pressure before fluid can pass to the FRONT brakes. This should be a moot point since your car shouldn't have one. The differential valve is symmetrical looking and has sort of an H shape to the cutout. The combo valve (aka proportioning valve) has sort of a dog-leg shape. If, by some chance, you have the wrong valve, your front brakes will have less stopping ability than they would with correct valve.
The differential switch will turn on due to anything that results in the pressure on one half to be significantly different from the other. An internal leak (m/c), external leak, or excessive air (in one side of system) can cause this. Your pedal is hard, so it doesn't sound like air (pedal would firm up by pumping it) but it does sound like it could be a frozen wheel cylinder. It's also possible to have the wrong length push rod but I think that's more of a long shot.
If it was me, here's how I would begin (you should be able to complete this in less than an hour):
1. Check the brake fluid in the m/c. If one bowl is low, that may be a clue on which end of the brakes to look for problems. If either bowl is low, look for leaks at all connections, wheel cyls, back of m/c, etc. If the rear piston on the m/c leaks, it will usually leak out the back of the m/c, so be sure to check under the dash where the m/c passes through the firewall.
2. With the car up on stands, have someone cycle the brake pedal while you verify each wheel will rotate w/o brakes and that it locks hard with brakes. You should not be able to rotate the wheel with even relatively light pedal pressure. While you're at it, you can asses whether the shoes are properly adjusted...there should be a very slight drag from the shoes w/o any pedal pressure for properly adjusted shoes.
3. Pull the drums off and verify the shoes are all on correctly. It's possible to put the shoes on backwards...the shoe with shorter friction liner goes to the front. Any wheel with shoes on backwards will stop better in reverse and very poorly going forward. If you're unable to drive along and lock up the brakes, this is what I suspect is the problem.
4. Use a small screwdriver to pull the boot back on each side of each wheel cylinder. If any fluid drips out, you need to rebuild/replace that cylinder. If you see rust, that's also an indicator of a problem with the w/c. #2 should have quickly shown if you have frozen w/c.
If, at this point, you haven't found something clearly wrong, I would do a quick brake bleed. This is more to check for fluid flow than it is for removing air. The car could have a smashed brake line somewhere and this will help find it. Starting with the wheel farthest from the m/c, have someone pump up the pedal and apply medium pressure...briefly crack open the bleeder and observe the volume that comes out. You should get a clear stream with no air pockets. Observe that each wheel should produce approximately the same amount of fluid. Be sure to keep the m/c full of fluid. Any wheel that doesn't produce fluid or only a dribble has a problem. Could be as simple as a clogged bleeder screw. If both front or both rear have a small amount of fluid compared to the other half of system, you have a bad m/c or check for smashed brake line. The front wheels have independent lines, so this would more likely be the rear, since it has a single line that splits at the rear axle.
If/when you end up replacing the m/c, I suggest you go to o'reilly's web site and do a search specific to your vehicle. Pay attention to the details...the m/c bores are different diameter for drum vs disc/drum. Also, be prepared with the drum diameter and shoe width to know what wheel cylinders it should have (so you can choose the correct m/c). I would have no problem with the best quality from o'reillys or autozone, but you'll generally get a little more knowledge from somewhere like Napa. I've checked, and many of the parts sold by these 3 come from the same source. On these older vehicles, I've had a lot of problems with the counter guys getting me the right parts.
Good luck and let us know how this turns out.
