Improper master cylinder, proportioning valve, or mixed up plumbing, and as a result something is holding a little pressure in the line? My first thought is a residual pressure check valve built into the master cylinder, and intended for drum brake system, but that is just a guess.

When it does this, can you crack the bleeder screw and get any fluid?

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-