Master Cylinder Bad?
Thread Starter
Melting Slicks
Joined: Mar 2004
Posts: 3,472
Likes: 18
From: Lake Ozark, MO
Master Cylinder Bad?
Power brakes. Car has been sitting for 2 1/2 years. Bled all calipers using the "gravity" method. Took a long time but tons of air out before the line cleared.
Results: pedal is flakey... good and solid about 1 out of 6 pumps.... the rest of the time it just bottoms out on the floor. With the reservoir cap off I noticed everytime I push on the brakes a small geyser off fluid/air would bubble up out of front and rear reservoir.....is this normal or indication that the MC is bad?
Results: pedal is flakey... good and solid about 1 out of 6 pumps.... the rest of the time it just bottoms out on the floor. With the reservoir cap off I noticed everytime I push on the brakes a small geyser off fluid/air would bubble up out of front and rear reservoir.....is this normal or indication that the MC is bad?
Last edited by Hvymtlc5; Jan 23, 2006 at 10:02 AM.
Melting Slicks
Joined: Oct 2004
Posts: 2,662
Likes: 8
From: Knoxville Tennessee
My guess is that the brakes are not completely bled yet. Need to get someone to help you; one to pump the pedal and the other to bleed one caliper at a time over and over untio you get a consistently firm pedal.
Bernie
Bernie
Thread Starter
Melting Slicks
Joined: Mar 2004
Posts: 3,472
Likes: 18
From: Lake Ozark, MO
Fluid is clear. Drained about 1 1/2 quarts with gravity method. I'm guessing the MC because of the inconsistancy in the pedal. I'll have pressure on one pump sometimes and on the others it sinks to the floor.
Pedal is better with engine off...will soften and go to the floor with engine running.
Pedal is better with engine off...will soften and go to the floor with engine running.
Last edited by Hvymtlc5; Jan 23, 2006 at 10:02 AM. Reason: addition
Instructor
Joined: Aug 2005
Posts: 104
Likes: 5
Got almost the same problem on a '77. Installed new master cylinder, and bled brakes at least 6 times. Still have intermitent problem... Sometimes brakes work fine and somtimes pedal goes to the floor. I have decided it is either a bad (new) master cylinder or the booster is acting up occasionally. Any thoughts?
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Melting Slicks
Joined: Sep 2004
Posts: 2,655
Likes: 19
The little geyser is normal.
The booster makes the pedal easier when assisted by engine vacuum. It requires engine vacuum for it to work.
Still could be master cylinder or caliper seals causing air leak. I've seen A LOT more caliper seal failures than MC failures, so that's where I would start. VB&P O ring kit if you've got S/S lined calipers.
The booster makes the pedal easier when assisted by engine vacuum. It requires engine vacuum for it to work.
Still could be master cylinder or caliper seals causing air leak. I've seen A LOT more caliper seal failures than MC failures, so that's where I would start. VB&P O ring kit if you've got S/S lined calipers.
Race Director
Joined: Sep 2004
Posts: 11,874
Likes: 263
From: Arlington Va Current ride 04 vert, previous vettes: 69 vert, 77 resto mod
Originally Posted by Hvymtlc5
COMP - what's the deal with the engine running or not? Does the assist put more pressure on the MC causing it to "fail"??
Team Owner
Joined: Sep 2003
Posts: 88,393
Likes: 2
From: eville in
Originally Posted by bobs77vet
the booster will always amplify the problem with the motor running or just afterwards until the vacuum bleeds off....you should have firm pedal with both
Instructor
Joined: Oct 2005
Posts: 113
Likes: 0
From: Melville, Long Island NY
Originally Posted by Garys 68
The little geyser is normal.
Are you getting any fluid leaks that would indicate a leaking caliper(s) or perhaps a corroded brake line?
I recently did an MC and calipers on my 79 because the system was shot.
If you go with a new MC (less than $100) make sure you take the time to do a thorough bench bleed. It will make bleeding the system a whole lot easier and faster.
BTW, the first new MC I got was defective out of the box, got a second new one, went with AC Delco unit, no problems.
Good luck.
Thread Starter
Melting Slicks
Joined: Mar 2004
Posts: 3,472
Likes: 18
From: Lake Ozark, MO
Looks like the right rear caliper IS leaking...didn't see it before..small trail down the inside of the rim...I'll get that repalced this week...maybe I can finally drive it with the new motor.. 
Burning Brakes
Joined: Aug 2005
Posts: 1,126
Likes: 0
From: Hudson Falls, N.Y. 76 Vette Modified L-48
Originally Posted by BerniesVette
My guess is that the brakes are not completely bled yet. Need to get someone to help you; one to pump the pedal and the other to bleed one caliper at a time over and over untio you get a consistently firm pedal.
Bernie
Bernie
Gauging that it sat for 2 1/2 years, I'd get a helper and do the pump 'em/hold pedal, bleed one caliper at a time, in the right order, twice around (or in your case maybe more), with a clear plastic tube attached to the bleeder and the other end in a mayo jar with about an inch of fluid in it, until "NO" bubbles can be seen...not even little tiny ones...just fluid...I am slowly learning that vette brakes are a very strange bird...but I am also learning how to deal with them...that's my 2 cents...