I have 72 OEM style power disc brakes on my car, complete with a proportioning valve, split master cylinder. Car stops great, does not pul to either side and is a vast improvement over the all drum previously. Problem is, no matter how hard I try, I cannot lock up the front brakes. Pedal stays firm and does not droop during stops. Read an article in one of the car mags where a guy had a similar problem and it was cured by removing the plug where the brake warning sender goes and reinserting it. Bled off air or something? Tried that, no go. Any suggestions? Autocrossed the car last fall and never had either the front or rears lock up, but car performed great. (At least it beat a stock Model A!) Any ideas or suggestions?
Gary, there is a hold off valve in the combination valve. Because disc brakes are instantaneous in their response, the hold off valve holds off pressure to the front brakes until there is a certain amount of pressure built up in the rear. Drum brakes need to take up clearance between the shoes and drum as well as overcome spring tension, so they are slower acting. That is why you can't lock up the fronts. That is a good thing, because once the front wheels are locked, the car will not steer. That is the reason for anti lock brakes, to keep the wheels turning, so that you can stand on the pedal and steer at the same time. Since the car stops great, I say you don't have a problem. Don't fix it if it ain't broke.
I thought 72 does not have the hold off valve? Or is the hold off valve not attached to the booster like the '70 has?
You can always install a residual pressure valve in the rear line. They come in various pressure ratings. Usually a 10 psi valve will be enough to hold the rear shoes to the drum (without dragging) so there is no delay in rear breaking. As Larry pointed out locking up the front brakes is not desirable, ..... neither is locking the rear brakes. It's all a balancing act. Personally I would prefer to have the ability to lock all four evenly and then adjust my driving habits accordingly without locking up. That way at least I know I'm working the brake system to its fullest. https://www.summitracing.com/parts/wil-260-13784 Larry
1971 and up disc/drum cars have a combination valve. It combines the features, proportioning valve/brake warning sender/hold off valve. The 70 and earlier have the hold off valve separate under the MC.
Thanks everyone, specially Larry. Great to think you have a problem and find out you don't. One less thing to worry about.
Bluzilla, Where in the rear line do you install residual pressure valve? Before the splitter on the pumpkin? Thanks , Jim
Jim, I install them before the rear axle branch, ... anywhere in the main rear feed line (after any valves). Larry
