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Help! C3 - Master Cylinder reach chamber drained + spongy brakes


New member
Jun 15, 2020
Pacifica, CA
1977 C3
Hi there! Brand new Corvette owner here, looking for help to build my technical skills. First post so let me know what I do right or wrong.

I have a 1977 C3. The other day I noticed my brake pedal was spongy and I had to depress it all the way down to get any purchase. I checked the master cylinder and the rear chamber was fully drained. No leakage around the MC. MC was replaced 1.5 years ago by previous owner (who kept the car in amazing condition)

Found brake fluid spattered all over the rear drivers side wheel. The line is still attached to the caliper but doesnt seem to be leaking. I am taking the wheel off today for further inspection. Any advice for a newbie? Ill post pictures and updates as I go. Looking forward to picking peoples brains as I work on my new baby :)
Welcome to the Corvette Action Center

The caliper piston seals are leaking. More than likely you will need to get rebuilt calipers for the rear. Most rebuilts are stainless steel sleeved so they don't rust again. In rare cases the caliper bores are good and just the aluminum piston has corroded allowing fluid to leak past the inside of the seal. In this case the caliper can be rebuilt at home with new pistons and seals. Replace pads too as they are ruined by the fluid contact. If the rotors are not grooved or excessively worn they can be either turned or used as is after cleaning. Given it's age replace the rubber brake hoses and also look at the short hard line on the caliper. This stuff is all readily available.

Update - calipers + pads + new brake fluid (front and rear) has me stopping much better. It still feels a little spongy and I have to depress the brake pedal almost all the way down before the brakes engage but when they do it stops great.
Did you use DOT 3 fluid? I have always had some trouble getting a correct pedal on a C2 or C3 until I bought a pressure blender. If you aren't with your pedal travel you can probably borrow or rent one from an auto parts store.
Used Dot3, gravity bled, and pressure bled. The seal might not have been very good when pressure bleeding. Any advice on what to use to create a better seal?
That flat top plate is hard to seal. I went to Harbor Freight and got 2 big C clamps. No leaks after that.

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