rx7learner

After shutting off the car I get a hiss sound underneath the bee hive maybe around the vac booster area. I'm guessing a vac leak on the booster??? but while it's hissing I rub my hand all around the the booster and lines and dont hear the sound change?

It only lasts for about 5 seconds after shutting off the car. Would anyone have any ideas on what it could be?

IanS

iTrader: (4)

Could be a noise the coolant makes. Also could be A/C if you still have it. I know I have had cars make hissing noises before and found out it was one of these things.

I'm usually slightly deaf after driving my 7 (exhaust and stereo) so if mine makes it or not I am not sure.

steve84GS TII

If you have A/C and it works,Id say thats the culprit.....very common,nearly all cars do it.

If not,try pumping the brakes quickly 3-4 times right after turning off the key.If the hissing no longer happens,then its the brake booster or plumbing.Pumping the brakes will deplete the resevoir of vacuum quickly and make the slow, leaking hiss go away much faster.

Boswoj

Make sure that is where it is coming from. If you have side seals going bad, it pressurizes the oil and it will hiss around the dipstick until the pressure equalizes. When you race a rotary you learn what a lot of noises mean in a short period of time ......

rx7learner

Its not around dip stick... Thank God.

However when I pump the break pedal it goes away (thanks Steve).

Cant find where the heck its leaking from though. Gonna get a spray bottle with some soapy water.

rx7learner

Yep.... boosters gone bad... darn thing looks fairly new.... if its a small hole think I could JB Weld it?

steve84GS TII

Its negative pressure (vacuum) so there wont be any bubbles.

I wouldnt trust my brakes (or anything else) to JB weld.Be sure to check the hoses and the check valve that comes from the intake manifold to the booster.
Its not common for the case of the booster to fail,its steel skinned after all.If its a seal or the diaphram,youll just have to bite the bullet and find another.If its the plumbing or backflow valve,thats an easy fix.
At least you know the booster could possible fail.Although there is a definate safety factor to consider,the car could still be driven without power brakes,youll just have to push harder to get the same braking effort.Knowing the booster is bad can keep it from surprising you one day, when you expect the power brakes to work,and they dont.

rx7learner

Thanks... I confirmed the hose isnt the source of the leak, when I sprayed around where the MC bolts to the booster I heard the liquid being sucked in... maybe that confirms a leak in a seal or diaphram... because I'm not sure where the check valve is... but I will look it up in the FSM.

Seems like the going price for a reman is about 70-100$

Thanks again Steve.