Ben's75

I'm having a rather inexplicable problem.

I just rebuilt my rear calipers this weekend. I reinstalled them and bled them thoroughly - two person technique. The brake light on my dash is on (has been for a while) and I still have NO rear brakes. I don't understand how this is possible. Obviously, fluid is flowing well enough to bleed air out, but the pistons don't move at all when the pedal is pressed.

Could this be my proportioning valve (I think this is the right term for the unit in 1975)? If so, I think I'll just remove it. I've inspected the lines, and there are no leaks. The exterior of the valve/unit was a little wet, so it may be leaking a little, but this still doesn't explain why I have NO rear brakes :mad. Any suggestions?

Thanks
Ben

68427

My guess is that the little piston in the small device below the master cylinder is stuck to one side. When differential pressure occures between front and rear braking circuits, the piston moves to one side and completes a contact - thus lighting the dash light. Try opening the device and centering the piston. This should allow fluid to flow to the rear calipers and turn off the dash light.

Mike Ward

Master cylinder needs a rebuild- internal leak. :cheers:

NHvette

Once all the air is bled out, then the piston in the switch is re-centered
by pushing down firmly (stomping) on the brake pedal. If there is still air in
the rear of the system, this will not work. Sometimes jacking the rear of the
car very high will help to get the air out of the main rear line during bleeding.

All C3s have a switch. The 'proportioning' of the brake power is done with equal
pressure front-rear and different caliper piston areas in the front/rear calipers.

OK - just read Mike's comment above. He may be onto something there, too.

:seeya


[Modified by NHvette, 6:55 PM 2/17/2004]

norvalwilhelm

My solution to this problem has always been to remove this warning light. It makes the brakes feel so much stronger and cures alot of problems. I don't have this valve in my vet or mustang.
I do not drive in the rain also.

LiveandLetDrive

Where to get an adjustable proportioning valve? Cure the problem, and not have the front/rear inbalance that you would without anything there.

-Chris

norvalwilhelm

Our calipers already have smaller pistons on the back then on the front. I have hydraboost and no proportioning valve and I find that all 4 seam to lock up at the same time.

Duke94

The light stays on because you have NO REAR BRAKES. There's nothing wrong with the proporting valve. Either the M/C is bad or you are sucking air past the seals in rear calipers. Yes, I know you just rebuilt them. Did you replace the caliper boots as well? Make sure they seal well on the OD press fit area. If not, you'll never bleed the brakes. O'ring style calipers solve this problem.

Ben's75

I don't think it is leaks in the calipers - I was being very paranoid. The master cylinder is brand new. I thought maybe something had gotten stuck in the proportioning valve - the valve thought that the rear brakes had a leak (it did at one point) and "shut them off" - and then got stuck in that position. Yes, no?

Norval, do you mean to remove the valve? You said to disconnect the warning light, and I don't think that would do much good. :lol:

Ben

Mike Ward

Now I'm even more confident that it's the master cylinder :cheers:

Fleetwood

I also had this problem,turned out the master cylinder was seized.Even though it had been overhauled,& fitted with a stainless steel sleeve.

Neil.

norvalwilhelm

Norval, do you mean to remove the valve? You said to disconnect the warning light, and I don't think that would do much good.

Of coursre disconnecting the light does not good. I was referring to totally removing this valve. It is not a true proportioning valve anyway. I plumb the front of the master to the front brakes and the rear master to the rear brakes. This gives you 2 completelely isolated systems. If one fails the other doesn't know it. The pedal still feels fine.
Your warning light might be stuck.
I have no use for that assembly that warns you one end is out. Try stopping after you loose one brake line and the pedal goes to the floor. Try the same thing with out this valve and the brakes still stop very well.

Ben's75

Well, I guess my plan should be to bench test the MC, and I think it that checks out, I'll just remove the valve. I've been reading a lot of the archived posts, and whether it's actually malfunctioning or not, I think I'd rather go without.

GaryS

There just may be a reason that the General paid a few bucks to engineer that switch and install it in the car! ;) Imho, that is like unplugging the carbon monoxide detector because it keeps going off.

You have a problem. Fix the problem and the light should go out. Everything else is a bandaid.
Gary

Duke94

The valve does not "shut them off". It just turns the light on if part of the system fails. I would not advise removing the valve. It's there for safty reason (to detect particial system brake failure). If your master cylinder is new, make sure it's the correct one and that the M/C push rod is contacting the end of the piston without excessive travel. You are not creating any pressure in the rear system which is why the light stays on. You need to find out why.

norvalwilhelm

Has any of you guys actually removed this valve and then tried the brakes? I have twice and both times found a big improvement in the back brakes. They finally felt like they were doing their job. You could feel the added pull from the back.
Have any of you guys experienced a blown brake line/caliper?? I have in other cars and trucks and the pedal goes to the floor and you limp home. Sure the light comes on, big deal, your foot has already told you there is a problem.
Without the light/valve you will not have your foot to tell you anymore because the pedal will still feel ok, firm and the car will still stop fine. I have not experience this first hand because my brakes have not fail yet on the vet or mustang but from foot bleeding you will find as soon as one end is bled the pedal feels great.
GM/Ford/Dodge put them there for a reason sure but after experiencing a number of failures at critical time from other cars how good is it.
My ford almost instantly emptied the master cylinder since the 2 are connected, my bonneville would hardly stop, the pedal was on the floor.
Guys don't knock removing this valve until you actually try it. I have extensive tinkering experience/problems and in every case removing this valve/light improved the situation.
How hard is it. Experiment and learn.
Guys I have done 3 spring swaps in one day just to test different setups. What better way is there to spend a saturday then experimenting. Take it out, don't like it , put it back.

68raggtop

Had the same problem on a TR6 years ago. After bleeding I went back and had my wife push VERY hard on petal while I cracked open the bleeder screw in one of the front calipers. The valve finally broke lose (actualy cut off the fronts), went back to the rear and slowly did the same until I got the valve centered. :smash:

The Dude

Your '75 doesn't have a proportioning valve. Instead you have a pressure differential switch. It's just a dumb brass block. Inside is a slider that senses pressure in the front and rear hydraulic lines. When the pressure front and rear are equal, the slider is centered. if one line has more pressure than the other the switch will slide off-center, thus turning on the brake warning light.

My guess would be the same as others have said--an issue with your master cylinder or a leaking caliper or line somewhere. I think the key here is that you just rebuilt the calipers. I would suspect something isn't quite right with the rebuild. I'd start troubleshooting there and then check the lines and M/C. I don't think I can be of any more help than that without seeing them.

norvalwilhelm

I copied this from Tom454 in his post on making a pressure bleeder.
It is a very good post.

If your brake switch (proportioning valve) has been rebuilt and is functioning correct, you will have a hard time bleeding the brakes. I did several tests also regarding the switch and found that it slides off-center EXTREMELY easy when it is working properly.

In extreme cases (good working switch) I have found it necessary to open both a FRONT and a REAR bleeder at the same time so that the switch does not see a pressure differential between the front & rear circuits. This way, the switch stays centered and allows fluid to flow to both open bleeders at the same time. Even then, in a few cases, I have had to reset the switch by using compressed air backward through the line because it tripped prematurely(but correctly).

A properly functioning switch will slide off center, blocking the fluid path to the open bleeder if you only have one bleeder open.

So why do so many guys NOT have a problem bleeding their brakes ?? Ans... their brake switch is all gummed up & is not working at all, or, the switch reacts slow to hydraulic pressure due to sludge.

I have pics of the whole test procedure with the switch(e) disassembled, showing the slime deposits on the inside of the switch.

Tom 454 did a good job on this post and I just wanted to pass it along.
Hope you don't mind Tom :)

427V8

Well great. Just freakin dang great.
I just bought a brand new shiny combo valve from masterpower brakes for $150....

It has custom bent tubing and a pretty bracket too. Now I see that it's just another part in the pile O parts.

Not having the valve makes complete sense.

Having only makes sense if your customer base is basically brain dead and won't bring in their car unless there is a light on on the dash AND the car won't stop :mad