This sounds like a proportioning valve problem
I have a 1971 Olds Delta 88 Custom that my late wife bought new. Needless to say, it is a family heirloom. It uses front disc and rear drum brakes. Sometime back the Bendix dual diaphragm power booster developed a vacuum leak and I replaced it with a rebuilt unit. At the same time I replaced the master cylinder, which I bench bled prior to installation and then bled the brakes. Upon completion of the work, the brakes were never the same. It took tremendous pedal but I should note the applied brake pedal will sink when starting the vehicle, indicating the booster is functional.
After much studying of the factory service manual I can’t help but believe the hold-off valve portion of the combo (holdoff/proportioning) valve has failed (stuck). Is there any sure-fire way to check this? I would gladly rebuild/replace the valve if the parts were available. I have only been able to find reproduction valves for GM intermediates and pony cars.
I also have found part kits for rebuilding the combo valve but they do not contain the cup seal required for the hold-off valve portion. It appears new valves for the GM intermediate size models (Cutlass) will physically bolt in place of the original, but I also know that each valve was specifically calibrated for a particular model. I am assuming the internal bias springs are probably different and the remaining hardware would be the same. If so, could I possibly build a “Frankenstein” valve by robbing parts from one valve to convert the other?
I hesitate to buy a salvage yard valve as there is no telling what I would get, in addition to possibly contaminating my brake system with who knows what.
I am hoping you or your readers have some ideas as I am all tapped out. If it is not the valve, what is it?
I believe you have determined the answer to your problem. It sounds like the proportioning valve is not working. Disc brakes require a proportioning valve because while your rear drum brakes only need about 450 pounds of pressure to actuate effectively, the front disc brakes need in the neighborhood of 900 pounds for the same job. The only way I know of to check this is with a pressure tester that can handle that kind of load. Unfortunately, very few of us possess such a device.
But you have effectively eliminated the possibility of a defective booster as the cause, so unless the master cylinder is bypassing or defective, it would seem that it has to be the proportioning valve. A good source of brake components will certainly have a new or rebuilt correct proportioning valve for a car of that vintage for a nominal price, so I wouldn’t go to the trouble of rebuilding it myself. However, if you want to do so, the same parts stores can probably supply a kit. I have provided the following source for brake parts before, but I offer it again because I have dealt with them many times and I know they are reliable and knowledgeable. Ask for Vince at: