Hello friends across the Atlantic!

Here I have a 1931 FORD A streetrod ( V8 Chevy and transmision TH300) on which I have a problem of braking until the insoluble!
When I activate the brake pedal, the vehicle stops perfectly for about 10 or 15 braking (or more). Hazy at the 16th brake (or more), the brake pedal goes to the bottom, the red light lights up and there’s almost no brake!
I first replaced the rear wheel cylinders with brake shoes, then the brake calipers seals, the master cylinder https://www.rockauto.com/en/moreinfo...pt=1836&jsn=16) , then the brake proportion valve https://www.summitracing.com/parts/clp-pv-2, then all brake lines (rigid and flexible).
I bleed the brake circuit without any particular difficulty, by putting the bleeder tool on the PV2 valve. Despite the exchange of all these parts the problem still remains (problem of pressure in the circuit)... !
There is still the brake booster to replace, but for me the symptoms are not related to this problem.
Do any of you have any idea ( diameter of rigid pipes, !? ...) I don’t know what to do anymore.

Thank you very much for your help.

Jef
France