Hi everybody, I think I got closer to the sticking pedal problem:
This is what I did today:
1. I narrowed the rubber hose between the brake booster valve and the filter with pliers, so that less vacuum was provided: At idle (700 rpm) the brake pedal didn't stick, but the booster worked. I tried this several times pushing the pedal very hard, it always came back.
2. I reved the engine at higher rpm (about 2000), the pedal stuck once and a time.
3. I fixed a 6 mm (outer diameter) copper hose like used for brake lines between booster valve and filter (see pic) and test drove several miles, used the brake until the wheels blocked, braked very hard several times, also in gear L2 and L2 (for higher revs), THE PEDAL ALWAYS CAME BACK!!
So my conclusion is: Your first guesses were right, it's the (new) brake booster which somehow doesn`t get along with the vacuum provided by the engine. Am I right?
Nevertheless I ordered a new brake booster which will arrive next weekend.