I did a search but didn't come up with anything related to this problem I have a buddy who has a 62 Impala SS and is having brake problems. It has a firewall mounted CPP booster/master cylinder with proportioning valve, disc front and drum rear. When driving the car the brakes do not stop the car very well until he has driven 4-5 miles, then they work fine. After sitting awhile the same thing happens until driven 4-5 miles. Vacuum is 20-21, all the lines are new, the MC was bench bled and the system bled multiple times. MC bore is 1" diameter and the booster is an 8" dual diaphragm. Any additional info needed? Any idea of what is happening? Thanks for any help on this. LE
What is the vacuum when the engine is cold? maybe the choke is acting up? Might want to be more specific about "the brakes do not stop the car very well". Is the pedal hard to push? or do the brakes just not seem to have much friction?
i was thinking low vac cold, might be cam to big would build vac running down the road but at ilde and cold with no reserve or check valve might bled off vacuum? lots of power brake problems concerning 8 or 6 inch boosters these days
At cold start the engine shows 18" of vacuum so I think the vacuum is ok. We'll check to see if the check valve in the booster is working. The choke seems to be working properly so I don't think that is an issue. If you have your foot on the pedal when starting it does drop a little as it is supposed to do. However, if you move the car when cold the pedal goes almost to the floor if you try to stop and you have to really push on it to stop. The engine is just a stock 350 so it does not have a big cam. Thanks for the comments so far. Larry
The booster/MC/proportioning valve were bought as a unit from CPP. The proportioning valve is supposed to have a residual built in but I can't verify that. I guess we could add a residual valve to the line to the rear drums and see if that solves the problem. I'm not sure I understand. You mean leaking brake fluid past the cylinder??
The master can byp*** internally. You have no external leak, the fluid gets spit back into the reservoir. So you lose the pedal to the extent where the master bottoms out and most times on the next pump it works perfectly. I never saw one that byp***ed 1st thing in the day, but it is possible. You can do this test with the car off. Sit behind the wheel and play with the brake pedal to see if you can make it drop. We used to call it tickling the dragon. Give it a little pressure, then more, then a series of short stabs at it. I could usually make a suspect master do it after a couple of minutes.
I'm guessing that the proportioning/distribution valve could have only one residual valve built in, intended for the rear drums only. Is it possible that the prop valve was plumbed incorrectly? I ran into this same issue several years ago and the problem didn't go away after the first few minutes of driving. At any given time the pedal might drop and leave you with just the rear brakes, I believe. A second or third stab at the pedal brought back full braking power. It kept you on your toes! A new master cylinder was the permanent fix.
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