No brake pedal?
Thread Starter
Heel & Toe
Joined: Jun 2006
Posts: 18
Likes: 0
From: Falls Church VA
No brake pedal?
75 corvette. Brakes did work fine, then started to get mushy. Replaced the calipers and the pads. (I actually also did the a-arms, springs, spindle, ball joints, rotors, hubs and bearings) I've mighty vacked the system a couple of times (Front and rear) and have sucked out alot of fluid. The pedal continues to drop to the floor. Did my booster somehow quit working as the car sat for two months?
thecasper
thecasper
Pro
Joined: Apr 2005
Posts: 618
Likes: 0
Do they start out good after you bleed them, and then get mushy, or are they mushy all the time? Did you measure the rotor run-out? If it's beyond spec, the rotors will wobble enough to pulse the pistons and that causes air to enter the system, thus mushy pedal. Get a dial gauge and check the run-out.
Team Owner
Joined: Aug 2006
Posts: 24,125
Likes: 15
From: Columbia Missouri
Sounds like runout is sucking air back in after the system is bled, and you lose the rock hard pedal.You can test your booster however, to put you at ease.
With the engine off, depress the pedal until it gets hard. It should only take a few pumps. If it gets hard and stays...the booster is good. If it never gets hard then you have a leaky booster. Then, start the engine, and the pedal should go soft again and return to where it sits when you stand on the brakes.
Thread Starter
Heel & Toe
Joined: Jun 2006
Posts: 18
Likes: 0
From: Falls Church VA
Thanks for the reply. I 'm not familiar with runout. The rotors and pads are both new and everything seems snug fit. I can tell there is fluid at the pistons. There does not appear to be any pedal pressure build up without the engine running.
Thecasper
Thecasper
Last edited by thecasper; Apr 3, 2007 at 08:13 PM.
Team Owner
Joined: Aug 2006
Posts: 24,125
Likes: 15
From: Columbia Missouri
Well the problem may bot always be the rotors...but the wheel bearings. If the bearings are bad or out of tolerance, the rotor can still wobble as if it were untrue as well. This will in turn cause the pistons to suck air in. There is still fluid at the pistons...but air int he system even in small amounts will result in a mushy pedal.
Team Owner
Joined: Aug 2006
Posts: 24,125
Likes: 15
From: Columbia Missouri
Okay, that's good though because it continues to narrow things down.
How about the master. Is it new or rebuilt?
A bad master may also let air into the system. The booster contains no fluid so the only thing a bad booster will do is stop helping. The master can go bad, and cause problems.
Thread Starter
Heel & Toe
Joined: Jun 2006
Posts: 18
Likes: 0
From: Falls Church VA
There is no pressure build-up on the pedal at all. It goes to the floor pretty much every time. Mastercylinder is old/maybe original as well as the booster. Seemed to work ok when I parked the car. Any other way to test the MC or booster.
Thecasper
Thecasper
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Team Owner
Joined: Aug 2006
Posts: 24,125
Likes: 15
From: Columbia Missouri
Is it hard to press the pedal to the floor at all? If not, then you might be in the market for a master rebuild kit. It's only around $30, and will take you about an hour from the time you take it off to putting it back on.
You'll have to bench bleed it, and then bleed the entire system...but it'll most likely solve your problem.
