Strange brake problem

[wmhjr]

A very good friend who has helped me a lot on my project has a very nice '53 Ford F100 truck. It's got a '77 or so Monte Carlo front end, a SBC, '77 Monte front calipers, rotors and spindles, and a '77 Granada master cylinder and brake booster. Master cylinder is on firewall. Large reservoir goes to T, then to left/right caliper. Small reservoir goes to prop valve (new) then to rear drum brakes (T over rear end).

The truck has been driven extensively with no issues. However, not long ago the master cylinder failed, and had to be replaced. The replacement worked, but it ended up leaking (bad quality lid on new part). Now, the new replacement doesn't leak but it's doing some strange things.

1) The brakes have been bled too many times to count. When the engine is not running, there is very firm pedal pressure, and with the front end of the truck raised, it takes little pedal movement to apply brakes to front wheels.

2) When the truck is started, the pedal is no longer firm, and goes almost to the floor before engaging the brakes.

3) Obviously, if truck is running but vacuum line plugged and booster not connected to vacuum, it behaves exactly like when truck is not running.

4) Booster is not making any noise. No indications that it has failed.

Based on the troubleshooting I did tonight, I'm thinking that since the brakes seem to engage quickly when the booster is not given vacuum, the problem could not be the calipers, lines, air in the lines, or master cylinder. So, it has to be the booster, right?