Spongy brakes *sometimes*

[Helge Wunderlich]

I have this strange issue with my brakes:

The pedal is sometimes tall and firm, just as I want it to be, but
other times, it is lower and slightly spongy, just as if there was a
little air in the system.

The problem can come and disappear from one second to the other, but
always between brake applications. That is, the pedal does not sink
while I am on the brake, but if I release the brake and apply again,
it can change.

When low, the pedal goes about half way to the floor before the wheels
lock on dry road, so there is still plenty of braking power.

The amount of effort to obtain a certain brake force does not change,
so the booster system seems OK.

There is no hydraulic leak, neither brake fluid nor Pentosin.

Rear calipers, discs and pads are new (calipers are remanufactured). I
think maybe the problem started after I replaced the rear calipers,
but I am not sure. I have replaced the rear brake lines earlier, so I
think my bleeding technique is OK. I have tried bleeding several
times; I must have a couple of liters through by now.

The "patient" is a 1984 100. Hydraulic brake booster, discs all
around. WC engine (2144cc, 10V). No hydraulic suspension.

Is it possible the master cylinder can draw air and let it out again
somehow ?