A pair of residual pressure check valves may solve the problem.

I have used them on two of my dual circuit master, under the floor, conversions, and I have never had brake problems. I also has 4 wheel disc brakes, I assume you have drums?

The check valves may solve it.

But can anyone explain why the original master cylinder did not need one, and now they tell you if the master cylinder is under the floor you do need them? What changed between then and now?

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-