MK VII Brake problem

My 1957 mk7 has a curious problem braking: after a continued braking or 2-4 brake presses in a row pedal becomes extremely hard and brakes remain on then, a sudden kick on thee pedal releases everything till the next time. Also if I stop the motor everything returns to ‘normal operation’
I suspect on the springs in the servo unit, the three of them. This servo was rebuilt ten years ago because of this same problem that wasn’t solved (I don’t know what parts were replaced)

So, May I change all the springs inside the servo as well as the springs of the brake pads to ensure the return to the ‘release position’ of all the system, or could be other the problem?

I don’t have any experience with the Mark VII vacuum brake servo, but your problem could also be in the master cylinder, not returning all the way or the rubber cup blocking the fluid return port and thus not releasing fluid pressure in the wheel cylinders. It happened on my XK120 and I concluded that it was silicone DOT 5 brake fluid had caused the rubber cup to swell just enough to block the tiny port.
In yours it would be the RECUPERATING SEAL blocking the rear top port just below the letter B in BODY.

It could be. I’m using DOT 4but I’ll explore this possibility…
Maybe a not released fluid pressure prevents the plunger that opens the diaphragm valve to return to its rest position maintaining the valve open…

Pinch the vacum line to the servo. If the problem goes away then blame the servo. I had a very similar problem with the brakes failing to release until I tapped the brake pedal, then I drove the car for several months with the vacum line pinched until I could get the servo rebuilt.
P. Smith

Also check flexible hoses are not collapsing as fluid is drawn back causing a temporary hold of pressure until the flabby hose regains composure.

Finally solved:
The problem was the master cylinder,it was a mix between the original and a new one slightly different.
I restored (basically cleaning it ans replacing seals and the spring) the original one and, the most important: adjusted correctly the operating rod, that is, maintaining a clearance of 0.20 mm between the rod and the seal retainer and adjusting properly the brake pedal free travel (it was lengthening it).