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brake master cylinder overflowing!

jimmers

Well-known member
Joined
Mar 27, 2005
Messages
302
Location
Canada
Corvette
1990 Red Coupe 6 speed
I replaced my brake master cylinder a few months ago and now for whatever reason all the brake fluid will go into one reservoir and overflow. One reservoir will be completely full and seaping out of the seal while the other is half full. I've refilled it many times but i get the same result. Any ideas?
Thanks everyone
 
Well , my guess is it's the seal inside between the reservoirs. Why did you change the master cylinder , what was the reason ? And how long before the problem started ? And how's the braking ?
 
i changed the master cylinder because my brakes were very spongy. Its a brand new master cylinder and reservoir. The braking has been great since its been changed but the problem started about a month ago.
 
My master cylinder only has one resovoir. I know the older styles had to individual resovoirs for the front and rears. Why is mind different then yours?

Could you have a brake pad that is dragging, causing the fluid to get really hot and expand or something. I know if you heat the brake fluid enough you can get moisture in the lines creating that spongy pedal you were talking about.
 
well the spongy feeling is gone. All the brakes line were bled when the new master cylinder was installed. I have seen many pictures of vettes with just one resovoir but for whatever reason mine has two. This is the first time it has been replaced and both master cylinders had 2 resovoirs. Im stumped.....
 
vetteboy86 said:
My master cylinder only has one resovoir. Why is mind different then yours?
The C4 Corvette went from one reservoir in 1984 to two in 1985, then back to one reservoir in 1987.
 
Ken said:
The C4 Corvette went from one reservoir in 1984 to two in 1985, then back to one reservoir in 1987.

Maybe that's part of the problem... wrong replacement part???

Did you do the work yourself? If a shop did it, I would take it back and have them check it out. If you did it, perhaps you hooked up the lines in the wrong location. With 2 reservoirs, one may be larger than the other to compensate for more line capacity. :confused
 
i did the work myself. I got the part from a GM dealer. I assumed it was the right part because like i said the car originally had 2 reservoirs.
 
My '90 has two resevoirs also. Replaced the brake booster and master cylinder when the booster failed. Replacement also had two resevoirs, no new "cups" had to use the old ones.


:w
 
jimmers said:
I replaced my brake master cylinder a few months ago and now for whatever reason all the brake fluid will go into one reservoir and overflow. One reservoir will be completely full and seaping out of the seal while the other is half full. I've refilled it many times but i get the same result. Any ideas?
Thanks everyone

Go to "howstuffworks.com, search "master cylinder", go through the animations. It just might give you a clue on what's wrong. Something going on between the primary and secondary piston. Hope it helps!
 
Robertwav1 said:
Go to "howstuffworks.com, search "master cylinder", go through the animations. It just might give you a clue on what's wrong. Something going on between the primary and secondary piston. Hope it helps!

that a really good idea! I never thought about that. Thanks
 
You see , I do not know the exact construction of a vette master cylinder , but on my other car I changed it because it was not braking good . there are seals in there , kinda which have spring in them , and those got old , and got changed . But there is some trick in installing them , I guess you have to wetten them with brake fluid or oil them . Because if you don't , ... mine started leaking again , braking was bad , you know , pressure does not hold cause fluid goes through seal back into the reservoir .
So , I bought the new one , installed it , and again it is not that braking i would like to have . I guess if you install them on dry , those seals get damaged or something .
Or maybe bore of the cylinder or piston have openings in them .
You do have a new part , that's good , but maybe that part was laying onna shelf for long ? That's my best guess . I actually need answers onna same questions :)
 
I think you need a new master cylinder. The master cylinder has a proportioning valve built-in. Since the front and rear brakes are isolated circuits, the only place fluid could transfer from one circuit to the other is the proportioning valve. You could confirm this with dye, but I do not know what kind of dye would be compatable.
 
Ken said:
The C4 Corvette went from one reservoir in 1984 to two in 1985, then back to one reservoir in 1987.

then back to 2 in 88

I had the SAME problem with my 89.. I finally swapped to MC from a 99 F- body
 
jimmers said:
but its brand new!!

Right. Everything GM makes is perfect?

You are saying that fluid is moving from one reservoir to the other? If yes, then this is SO simple. I don't know if it is a law, but every automobile made for the last 50 years or more has isolated circuits for the front and rear brakes so that a leak in one circuit will not cause total brake failure. The only place the fluid from both circuits comes close to each other is at the master cylinder and brake proportioning valve. Since the proportioning valve is built into the master cylinder for your vehicle, your NEW master cylinder has an internal leak. This is the only way fluid could move from one circuit to the other, unless someone is sneaking out in the middle of the night with a syringe.

Mike
 

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