Occasioanally, my Series 4's brake pedal pressure increase at no specific pattern or external factor at all.. Sometimes it gets heavy after a long hot drive, but this morning after start up the pedal pressure was so heavy for the first 10minutes and back to normal after that. It feels like typical scenario of not getting vacuum into the power brake unit but after inspection of the hose, it seemed fine. Could it be a collapse diagphragm or plate? Thinking of swapping the whole unit with the master cylinder to a series 5 type. Any advantage of doing that??

 

Could be internal damage in the booster, but you won't be able to verify it until another one is swapped in.

There are tests in the FSM (and maybe Haynes) to verify booster function- have you done those yet?

 

Too troublesome! Certain parts r cheap here so I guess I'll just try to do a swap but definitely rebuild it b4 going in.