When engine is at running temperature bakes initially work for a second, then pedal goes hard as if there is no vacum or power assist.seems to be ok when car is cold.has anyone any ideas please, as at present my mechanic is scratching his head, so im hoping to assist him in diagnosing problem.any help greatly appreciated🙂

 

Could be a vacuum leak. Check all vacuum pipes and joints for leaks by spraying Easy Start around them when the engine is running. Be thorough. If the engine note changes, you have found a leak.
The other possibility is a leak inside the servo, in which case it would need reconditioning or replacing. One way to check this is to pump the brake rapidly while the engine is running. If the engine goes to die, you have a fault in the servo. If the engine note changes only slightly it could be ok.
Try the above tests and please report back results for analysis.

 

do a smoke test to easily find any openings in the intake system

 

The check valve on your brake power booster is shot. Same problem here but O'Reilly's has it. It is a pain to get it out, but it is small and hooked to a vacuum line. You will have to look at the choices to the right one. It is less than $10.

 

hi
if it a diesel, then your servo failed
very common on diesel, non return fails and oil gets into the servo and the diaphragm fails
if its petrol then could be a split in the vacuum pipe, or again servo
cheers
Joe