Assume it is a standard stock brake system. The problem is air in system after sitting extended period of time. Sort’a like; worked when parked.

It is not likely the servo because it is a sealed unit. If servo leaks it is usually sucked into the engine.

If the car is stored in an environment where temperature swings are frequent, perhaps humidity high and Girling fluid is used, air and moisture will migrate past seals in the cylinders, including the master as corrosion develops.

Although air was extracted from rear cyl, it may have come from the master. It is not uncommon for master cyls to inject air as the seals wear and corrosion builds up in the bore from light corrosion deposits. Pumping the pedal tends to partially clean up the seals and make it functional again…..for a while.

Use silicone for vehicles used occasionally and parked for extended periods of time and these kinds of problems will not likely occur.