All wheels gets stuck on my MK 2 62 when i braking with a hot engine compartment. Hoses are OK.Air filter to vaccum reservoir clean.After 5-10 minutes they gets loose again.Must depend of something sticking. I doubt that the air valve on the vacuum reservoir can cause it. On the other hand perhaps a faulty air valve on the booster or faulty piston? The brake pedal feels stiff but goes back after depressing it .. Has anyone experienced this and why with a warm engine? Where to start??

 

I have had it before, it was the reaction valve on the booster. That sticks and allows vacuum to the booster which pressurizes the brake fluid. Remove the smaller diaphragm and the piston underneath. Piston #14 on the diagram in the part that sticks.

You might need slight pressure on the brake pedal to push the piston out, brake fluid will be behind this, so be prepared to plug the hole. The usual cause is rust in the bore, so clean it out and then reinsert the piston with some rubber grease and reassemble.

 

+1 for Jagboi--if all wheels are sticking it is the booster. Might be a good time to change all flex lines also but that would affect one or more of the fronts and the rear both sides.

 

On my car, the vacuum piston was sticking. It's possible to lubricate it easily by pouring a little brake fluid down the breather hose. Incidentally, the filter at the top of the hose should be dosed with a teaspoon of brake fluid at service time. The reason the piston was drying out was a constant flow of air due to failure of the one way valve that controls vacuum from the manifold.

 

Check if the master cylinder is coming back to its home position.
When stuck the valve to the reservoir is closed, and brakes get stuck.