ok i just recently replaced the cv joint on my 84 rabbit. when i backed it out of the shop, the brake pedal went to the floor. i had my dad take a look at it with me and it was obvious that the master cylinder was shot. brake fluid was squirting out from between the booster and master. i got a new one at napa and replaced it tonight. i bench bled it. my dad said it didn't act like they normally do but we got it to the point where no bubbles were coming out. we installed it without any problems and bled it after it was in the car just to double check it and no bubbles again. i backed it out and just moved it around (no driving on the road or anything) and you have to pump it up. it seems really squishy and doesn't really get hard. if you pump it up and hold it, after a while it starts going soft and moving down. some of the threads i read made this seem like it could be the booster. anybody have the same problems? any ideas?
Thanks.
well if the brake fluid was never changed, its probably super corrosive by now. best you can do is flush all the old crap out and maybe you will need to replace the booster as well
or at least the seals inside...
what seals? i was under the assumption that fluid couldn't get into the booster. but yeah i'm thinking about getting that old fliud out. the stuff that was in it is red and cloudy :shock:
easy way to tell if its the booster is to pump the brakes a dozen times with the engine off, that will use up the stored vacum. Pedal still squishy? Probably got air in the master or upper lines still. pedal not squishy with no vacum, but squishy with vacum is maybe the booster. Boosters do not fail often and when they do its usually a failure to provide any boost.
If the brake fluid in master was red and cloudy, it probably had water in it. So you should flush the system. Water in break fluid turns to steam when it gets hot. Steam can be compressed, so you loose your brakes, just like having air in them. Its also the water that makes fluid corrosive.
You can flush the system with gravity, just let it run out a bleeder on each wheel until it runs clear, keeping the master topped up of course. This might fix your squishy pedal as well; the air could be carried away. if not crack the connections by the booster and let them drip a bit, then tighten. pump pedal. repeat. This will help bubbles rise to top.
In tougher case, you need a helper to put light pressure on pedal while you crack lines/bleeders one at a time. ie: they hold light pressure, you crack connection, they stop pushing and hold pedal down until you tighten connection, then they let up. You will probably hear bubbles coming out. repeat until pedal is solid with engine off.
When you did the CV, did you have the brakes apart? Sure its back together right?
The fact that it leaks down after you pump it up is strange. it implies a leak somewhere so if there are no puddles of fluid under car to clue you in, maybe new master is bad. Masters usually hold pressure under high pressure and leak under light pressure when they start to go.
Was the master actually new or rebuilt? I have had rebuilts do what you are describing. Some rebuilders just hone the inside of the master cylinder out and install new seals without measuring the I.D. of the bore. If it's oversized, the fluid can slowly leak past even the new seals. If you have some plugs that will fit the exit ports on the master cylinder, plug them up and if the brake pedal still does the same slow drop you have a defective cylinder.
The bolts that hole the pressure plate to the crankshaft work great as plugs. I always carry a couple with me in case I blow a line or something and have to block it off.
thanks for the ideas. i'm gonna start by bleeding out all of the lines at the wheels, flushing out the fluid while i'm at it. the master is from napa and they've always had good quality in the past, but i'm not gonna rule out the possibility of it being bad. good news is i'll be spending all weekend working on it since it busted a motor mount today (yes i was driving it with the master like that :roll: ) i think that from reading this stuff, i can probably rule out the booster. BTW are the lines that go to the calipers supposed to run directly to them without going to a proportioning valve? seemed kinda stupid that they don't.