Hi from Germany! My Riviera has this problem: Sometimes (engine on) the brake pedal sticks after hard braking and comes loose again after a few meters of driving. Brake booster and master brake cylinder are new. I did the brake booster test (engine off, some steps on pedal until hard, engine on, pedal goes down a few inches) and this was ok. When the pedal is stuck (at idle) and when I remove the valve from the brake booster the pedal comes up! There is a lot of vacuum on the hose and the valve is ok. I exclude problems with calipers, brake hoses, because when I use the brake with the brake booster valve disengaged everything is ok (with the exception of course that I need much force to brake). I would very much appreciate every hint what I can do next and how to solve the problem.

Regards
Michael