When my car is running I can hear an air hissing sound near the brake master cylinder. When I shut the car off, the sound slowly fades away. I'm guessing there is a vacuum leak somewhere, but I can't really feel anything leaking around the big vac line from the brake booster to the intake manifold.

Also, after shutting the car off, the brake pedal feels very stiff. It's like when you pump the brake pedal a few times with the car off, but mine feels stiff the first time you pump the pedal.

I sure hope it's just a leak in a vac line and not the brake booster itself. Any thoughts?

 

Take and get yourself a vacuum pump and disconnect the vac line and pump it up.

It should hold vacuum obviously. Do you have the one way valve inline on the vacuum line?

My bet is the booster but it might be the hose itself.

 

I have a MightyVac pump, commonly used for bleeding brakes. Would that work?

I'm guessing I should disconnect the hose from the intake manifold and connect that hose end to the vac pump right?

Yes the one way valve is in place.

I just noticed this happening within the past couple of days. Weekend before last I did replace my rear brakes and then got on the brakes pretty hard while bedding the pads in, so I don't know if that could have anything to do with this current problem or not.

Also, brakes feel fine and seem to perform normally while driving.

 

If you adjusted the rear brakes out to the outer limits then your pedal would feel harder than before, go to Sears and buy the mechanics stethoscope, I swear by these things, pull the metal tip and diaphram off and use the hose to listen for your vacuum leak, you will find it. They also sell and hand vac pump with a pressure gauge, get this too.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Duane_in_Japan &raquo;</TD></TR><TR><TD CLASS="quote">If you adjusted the rear brakes out to the outer limits then your pedal would feel harder than before, go to Sears and buy the mechanics stethoscope, I swear by these things, pull the metal tip and diaphram off and use the hose to listen for your vacuum leak, you will find it. They also sell and hand vac pump with a pressure gauge, get this too.</TD></TR></TABLE>

Well the brakes feel the same as they did before the brake job when the engine is running and while driving. I'm guessing though that the pedal feels hard immediately after shutting off the engine because the vacuum is leaking out of that line (or out of the booster).

As for the rear caliper pistons, I turned it until I couldn't see it moving back anymore, but I may have continued to turn it a few more times before I realized that it wasn't actually moving back into the caliper anymore.

 

I would go for a quieter vac pump if you are listening for an air leak. When the car is off the pedal is suppose to feel firm. You don't have any vacuum helping it.

Do you have a smoke machine? you could fill the IM and see the leak that way too.

You did bleed the brakes, did you do them in the right order? With your pump I guess it would not matter. You want to do them...RR LF LR RF.

 

I only replaced the rear pads and rotors, so I did not open up the brake hydraulic system or bleed it.

I know the pedal is supposed to be firm when the car is off, but only after pumping it 3-4 times. The first time you pump the brake pedal with the car off, it usually feels normal. Mine feels firm the very first time you pump it with the car off, I guess because any residual vacuum in the system is leaking out of the line or out of the booster.

 

That is where the one way valve get's into the mix. It will hold vacuum for a little while with the motor off, probably just enough for one or two pumps.

Pull the one way valve and see if you can blow thru it both ways. They get gunked up to.

 

I don't think that's it, though, because I can hear the sound of air hissing while the engine is running when I put my ear near the master cylinder area.

Dang that little check valve is $18 from Son's Acura

 

Now I understand better, yes the vacuum leak will make the pedal hard right away. In case of an emergency and the engine dies, there is still enough vacuum in reserve to give you power assist brakes, those two or three pumps, yours is gone, forget what I said about outer limits on the brake adjustment, its good.

 

Check valve is good also, it only allows air to flow in the direction of the intake manifold.