I need your help to diagnose a power brake problem. I have a 62 GM product that had power brakes with a single master cylinder and drums all around. I had the brakes re-done, including a new master cylinder. After the repair, I noted that the brake booster wasn't working. I did the 'test' (i.e., engine off, foot on brake, start car) and the pedal pushed toward me as it was suppose to. I scratched my head and decided to put a new booster and dual master cylinder on it anyway. The booster is from Summit (8") and the master cylinder is a standard GM unit that has the same push rod style as the booster. The booster calls for 18" of vacuum which is what I have off the intake. I plumed in the vacuum canister that was in the car. The engine performs fine which would indicate to me that there is no vacuum leak. However, I still don't have power brakes. I do have manual brakes but it takes a lot of pressure to stop that big ragtop. The 'test' doesn't work on this booster. In fact, I think the pedal goes down a little bit when the car starts. What are your thoughts on my problem?
could be the wrong proportioning valve in the master cylinder, or wrong proportioning block for that master cylinder, maybe still poor vacume, big cam ? could still be a leak, try a new hose from the intake to booster, check one way vacume valve at the booster, if all o.k.? I'd say booster faulty.
There's no proportioning valve. This is an all drum car so none is needed. I'm measuring 18" of constant vacuum. Engine is stock 401. Will check valve and vacuum of entire system with pump. It just seems like a really big coincidence that the original booster and this one have the same symptoms. I can understand the original one failing; it's 44 years old.
I'm going to assume you are dealing with a Buick Nail Head engine since you said it is a stock GM engine and that is the only engine that comes to mind with 401 cid. It really sounds like a booster problem. The car came with origional Power Vac brakes and I am not sure of the ability to directly change this with a modern power brake booster. I have heard of people getting faulty boosters off the shelf in rare occasions also. Did you install a new master cylinder or rebuild the old one. By this time you could be getting some leakage by the piston arm in the master cylinder.
Try hooking the vacuum line direct from the intake to the check valve on the booster. Also, the pedal should go down a bit when you start the car...try it on your newer car. Another thing, how did you plumb the single line to the new dual master cyl?