I've taken my 70 to two different brake shops and they're both stumped. I have a hard pedal that brakes alright...but in essence - it's like plain old disc brakes without the power booster. They've bled the lines, checked for leaks, checked the master, confirmed master/booster pushrod length, booster function...all checked out fine. I'm faced with putting in a new master and booster and still ending up in the same place...only $200 lighter in the wallet (I have a spare booster). Any ideas what could be going on?
Did they bleed the brakes while depressing the button at the back of the factory disc brake hold-off valve?
I would check for proper vacuum to the booster. I once worked on a truck that would lose power assist. It turned out that the vacuum hose going to the booster would colapse after running the engine for a while.(I think it had been oil soaked form a leaking engine for years). A fairly simple check to see if the booster is doing anything to help is to pump the pedal several times with the engine off, don't let off of the pedal, then start the car keeping even pressure applied to the pedal. The pedal should sink about 1inch or so when the vacuum is applied to the booster. This will atleast tell you if you are getting any vacuum to the booster and if it is working at all. Also, you can check if all the brakes are contributing by putting the car on jack stands, then holding the brake. If the rear tires turn in gear with the brakes applied, the rear brakes are not applying. If you (or a helper) can turn the front wheels with the brakes applied, the front are not applying.
I've heard this a lot, and thought I may even have a problem with mine. So I looked in my Buick manual, and there is quite a bit about the proportioning valve and what it does. In the brake bleeding procedure I expected to see something about holding this valve in-- nothing? If it is critical- then why isn't it in the shop manual? Just wondering-
You aren't looking in the right place. It is in both my 70 and 72 Chassis manual. It is in the section on single piston disc brakes under the paragraph titled "Bleeding Disc Brakes"
Discussion 50-23 in the 70 manual. And, not that this is necesarily your issue, however, in diagnostics always go for the easiest solutions first. This one doesn't require too much work. I depress that thing (once a decade) using a carpenter's wood shim. Best of luck!!
Did you replace the front disc pads recently? Like the idea of the oil soaked vac line collapse. I have found that before too. Just some info on the disc pad replacment for you all. I do alot of Brake work. One day a buddy and I did 2 brake jobs side by side in my shop. He picked up 2 sets of Oriley's expensive disc pads {$40.00 or so} a set, for each of our jobs. Resurfaced the Rotors, packed the wheel bearings, also cleaned and lubed the brake caliper slides. We then took both cars on a test drive. They were both hard to stop. With the old worn pads it would almost throw you through the windshield. Hmmm, So we both went and removed the premium disc pads and returned them for their cheeper store brand pads. $19.00 or so. Test drove again and they were like they should of been. Perfect. I dont know if it was a bad batch or pads but consider, 2 different cars, 2 differemt mechanics, same pads, no power stop. we both changed the pads, fixed the stoping problem. So all remember this story. you never know.
don't know if you solved your problem yet. did you replace the master cylinder then the problem started? if so check the inlet connection to the front brake lines, it shouldn't have a check valve and spring in it, there only for drum brakes. you can pull it out with some needle nose pliers.
