Hard brake pedal under boost/Building boost

[Flicktitty]

So i was thinking about some of the problems the car's got while it is parked for the winter, I've been trying to figure out what is causing this issue and how to get rid of it. So i'm asking for help on here.

Car: 1989 Nissan 240sx SE
Toyota 2JZ-GTE / Aristo Auto/ Single turbo etc.

I've noticed when i do a pull and get into boost the brake pedal goes ROCK HARD and i have to pump the brake pedal once or twice to get the car to stop. Same goes for when i have the car in gear and i try building boost with my foot on the brake/gas trying to leave on boost the car makes 3-4psi sometimes 5-6 then the car starts moving as the brakes cannot hold the car.


I had this issue with the 1st swapped car, and again with the new swapped car. The new car has a 1 way check valve in the brake like (OEM Nissan) and new hose going from manifold to the booster. The car is on stock brakes (will be upgrading to Q45 in the next week or two) So how could i make the car have a stock like brake pedal when in boost, and have a brake setup that allows the car to make more boost.

Any ideas?

[codyace]

What you're descibing is actually kinda normal. When you're in boost, you're only available 'vacuum' is what is left in the booster...which isn't very much. When you first get on the brakes (or try to brake boost) it's going to be depleted of the needed vacuum to work (and give you a hard pedal).

I get that sensation on track on super long pulls, if I immediatly jump on the brake it's hard, but will become normal shortly (milliseconds) after. This is rare, but it does happen and that's the only explanation I've come up with to describe it (it feels like manual brakes)

[Flicktitty]

yeah i know it is "somewhat" normal. I've heard i could use a vacuum pump that would always have vacuum to the booster.

[180wanabe1]

should also be a plastic check valve to keep boost out, make sure its there.

[180wanabe1]

nvm just read the whole thing... you could go with chase bays booster delete...

[Kingtal0n]

they sell vacuum canisters for cars with big cams that pull very low vacuum.
but you probably knew about those already. Just thought i would mention it anyways.

Also, make sure you have the valve installed correctly, and check for leaks, because the pedal should not get hard suddenly just from going into boost, unless the vacuum in the booster was leaking out or getting pushed out.
even if you shut the car off, it should still store enough vacuum to mash the brake one or two times before it gets hard.

[Flicktitty]

see that's what i thought,that it should retain some pedal feel but it is hard after a solid street pull even at 14-15psi

[pacotaco345]

see that's what i thought,that it should retain some pedal feel but it is hard after a solid street pull even at 14-15psi

I doubt that check valve can hold 15 psi of pressure, and once there is no vacuum it will instantly become hard to press.

[Kingtal0n]

try letting it idle, and feel the pedal. Soft?

Now shut the car off. Let it sit a few minutes. feel the pedal. Still soft?

[jr_ss]

I doubt that check valve can hold 15 psi of pressure, and once there is no vacuum it will instantly become hard to press.

Think about what you just typed... A check valve is a one way valve and it has a seat. When there is vacuum, it's pulled off the seat. Opposite happens when you throw boost at it. The boost pressure will push it closed and actually seal tighter under higher amounts.

Flick, are you sure the valve is functioning properly? Have you tried removing it and hooking an air source to it to see if it actually "seals"?