While out for a what turned out to be a very short pleasure cruise last night, the brake pedal on my '70 LT1 (no power brakes) got spongy, and eventually went almost to the floor. The system was bled last month, by a mechanic I just started working with and trust. He replaced the fluid with DOT 4 brake fluid and said everyting looked fine. I have driven the car about 30 miles since bleeding.
The master cyl has about 2/3 fluid. The front diaphram is extended. The rear diaphram is partially extended at an "uneven" angle.
I looked behind both front wheels for leaks at the calipers, hoses, and on the tires. I also checked the rear although they are harder to see. I don't see any leaks.
The car had a body on resto in 1989, in Californina (now in Connecticut). I have an 8/23/89 receipt from R&J Corvette Parts, Anaheim, indicating purchase of mstr cyl (part #40090), brake calipers (#40082, 83,84,85) and core. I assume these are stainless calipers...
The stamp on the master cyl and cover are correct. The car has scored two Long Island Metro chapter top flights.
Any ideas what could be wrong?

 

Could be excessive runout caused air to be sucked into the calipers by the seals. Very common on these cars. Solution, fix the runout, and/or go to O ringed calipers (ie VBP).

 

thanks!

 

I think there is a procedure for brake bleeding starting on the drivers side rear if two bleeders do inboard then outboard the passenger side rear inboard outboard bleeders the drivers side front & then passenger side front....did this on mine 2 or 3 times & was very successful...also maybe there is a break in a line near the frame that is not detectable...yet! good luck

 

Be sure to look on inboard side of your wheels...

 

You really have to check the system well. The fact the car has numbers matching doesn't matter with respect to the system. You have a manual vette 4 piston system which is still pretty good today.
I agree you might have runout issues causing your problem but the only way to tell is to use an indicator on the rotors to find out. I don't know your guy but most of the "mechanics" in New Haven don't have an indicator or know how to use one. If you have a air pumping problem you can use the oring calipers but I rather find the problem and fix it. I just rebuilt a complete 72 brake system and didn't use O ring calipers, I got the runout to between .0005"- .002" at all 4 rotors.
If you have any questions PM me and I'll walk through the job with you.

Gary

PS here's some info on the grades of BF
http://www.tccoa.com/brake-fluid/