Losing Brake Power Assist At High Revs

[dr00]

morning kids

as the topic suggests im having some trouble with my brakes. ive got a silvia with an rb25det conversion, including a complete r33 brake system. this only happens on the track and doesnt cause a great problem but obviously id still like to fix it... if im at the top of a gear at the braking point at the end of a straight for instance and i jump on the brakes, the power is there at first and then bleeds away as if the engine has died. the brakes still work but theres just no assistance. i cant remember if the power comes back towards the end of the braking but it is back to normal by the next corner.

from what i understand there is a check valve in the booster that should stop air coming in. but if that wasnt working then shouldnt i lose all power assistance whenever im on boost? there is one vac line going from the plenum to the booster but there is also a barb connection on the side of the master cyl that has nothing connected to it... should it?

if that valve is the problem can i put a suitable one way valve in the vac line or should i just find another booster unit?

[Duncan]

yeah start with the valve.

basically when the throttle is open you don't have manifold vacuum, so you don't have vacuum assistance for the brakes. Normally this isn't much of a problem but they added a resoviour to store extra vacuum. the one way valve holds the vacuum in the cannister.

to check the valve pull it out and try and blow thru it, if you can blow through it one way but not the other it is working fine.

if not the valve it is probably the booster

do you left foot brake? and sometimes leave your foot on the throttle, even a little?

[dr00]

its cool i sorted it out. its definitely not the valve, coz i dont have a valve! ive ordered a standard R33 vac line from nissan because it has the valve inside the line, plus its only $23 and i dont have to get off my ass to find one from a wrecker.

[Duncan]

well I guess it kinda was not having a working valve

have you got it in yet? any better?