Hello everyone,

To answer to «rspears» the master cylinder is located in a classical height. There is no return spring at the pedal, it returns correctly thanks to the internal spring of the master cylinder. So there are no pressure regulation valves!
In response to "34_40" yes I have already bled the master cylinder to the workbench. Even if I had done it wrong from the start I think it is right now anyway because I did several bleeds and circulate a lot of fluid in the circuit. There’s no air coming out of all four wheels.
While reflecting there is a small noise of "toc' in the area of the booster push rod. I did not check the play of the rod against the master cylinder... need to do!
I checked the booster in 2 ways: Engine stopped 4 pedal strokes to empty the air and then started the engine, the pedal normally goes down by a third. Then I deconnected the booster’s depression hose, I drove the car and even the problem of pressure drop in the circuit (of course with a very hard pedal but it’s normal!)
Perhaps the problem of this car (Streetrod FORD A 1931) is that it’s equipped with parts of different cars (discs + front axles of 1969-72 Chevelle 4,1L- Monte-Carlo, 73-77 A-Body... etc) with a complete front-end axle of a 1968 Corvair chevrole; the rear axle comes from a Chevrolet CHEVELLE (GM N3969277 "10 bolts") with drums, the master cylinder equips the 1973 Ford Mustang, 1975 + Granada . There is a front/rear propotioning valve PV2 discs/drums (on which I fixed the plug to screw to bled).
I also changed all the piping according to the diameter already installed: 4.75mm on the front circuit, 6 mm on the rear circuit until (the rear rubber hose) then again 4.75mm from the center of the rear axle to each rear wheel.
In fact I replaced all the parts of braking identically by new ones with some difficulty because I had to search on the web the provenance of each of these parts! But perhaps there has been an incompatibility between them since the beginning when the car was built!? ... How to know!?

I always keep looking....... I want to thank you very warmly for your help!