Ive been helping out a friend with his brakes on his $150 90 STD civic. The M/C and brake booster have been replaced and the system has been bled and we are still having the prob. The brakes build pressure when the car is off, but once the car is started the brakes lose all power. We have checked operation of the booster check valve as well and it passed. Any help is appreciated.

 

maybe the vacum lines and check valves are bad when they come from the int mani to the MC....
if it works fine without vacum but then it goes bad.... theres something installed wrong....
i say check the valves again

 

We blew compressed air through all booster vac lines and we swapped check valves and got the same prob. All brake lines were checked also for any pinhole size leaks.

 

ttt