Brake pedal building too much vacuum under heavy throttle?

[Paper_Tiger240]

whats up guys, i recently swapped everything from my hatch to a new coupe shell

Both are 1989 240sx

Problem- During normal driving my car brakes work as brakes should work but I noticed that when under heavy throttle the brake pedal seems to stiffen up as if the car was off. Once you are pressing on the stiff brake pedal it loosens up back to the way its suppose to be.

Often causes a locking in the brakes because of the changes of brake pedal pressure.

its hard to explain in words but i hope you understand

Solutions- Vacuum leak? Bad Brake Booster?? Bad BMC?

Any input would help me out

[murda-c]

The one-way check valve in your booster vacuum line is probably stuck, and when you floor it the vacuum source drops to zero, and the bad check valve allows the pressure to equalize. If you take it out you should be able to only blow through it one way.



Replacing that should fix your problem.

[nieko]

I could be misunderstanding. But are you speaking of when you "left foot brake?"
Like tandeming and need to slow down a tiny bit so you push the brakes while flooring it?

If that's basically what you are talking about, or just flooring it and braking at the same time.

That is because you have no vacuum while flooring it, so the brakes stiffen up.

Disregard if that is not what you are talking about, hahaha.