The Stovebolt Site Forums The Parking Lot Big Bolts Unusual Brake Issues on 1956 Chevrolet 6400

I am working on my grandpa's 1956 Chevrolet 6400 (2 ton) and I have replaced the engine due to a cracked head in the original. The new 250 ci needs a few modifications to take the hydrovac lines, but it can be done. In the meantime, without the hydrovac hoses connected, I should be able to bleed the brakes and hold pressure without those vacuum hoses being connected, correct?

The Problem: After bleeding the hydrovac and the wheel cylinders, farthest wheel cylinder to the closest and no cylinder has leaks (three wheel cylinders have been replaced, one serviced, and the driver's rear has not been touched yet, but has not shown issues), the master cylinder holds pressure, then about 5 to 10 seconds later (when the fluid level is to the threads, it squirts out fluid at the vent hole during that time) it loses all pressure and goes to the floor again. I suspect that it might be a master cylinder issue or an hydrovac issue. Can anyone lend any expertise on this?

Thanks for your help!