I have a 1974 C-3 that I have had the brakes worked on twice and shortly after getting it back from the shop the brakes go soft again.  I can find no break fluid leakage anywhere.  Any suggestions?  Thanks ahead of time.
Nick  

Ok, I admit I have the bug for a corvette. (Actually I've had it for a
long time!)
Anyway, I'm interested in C4 (basically, because this is the price
range I can afford... around $12000)