funny idle, rpm goes up and down

[Carguy80]

I wonder what is the problem with my 92 v6 firebird.when my transmission is in p (park) or n (neutral) or i let the car run forward in street in low speeds,my rpms goes up and down.It does that sometimes when i let it run down the sreet,but it does it every time when the car is in park,and i push the brake pedal up and then i let it come back up.when i press the brake pedal down,nothing happens,but when i take my foot of the brake pedal,rpm goes down to about 500 and then 1000 and then they go back to the normal idling rpms,about 650-700. Sometimes it works ok,but if i turn off the engine,and then turn it on,and press the brake pedal down and after that rpms goes 500-1000,and then goes back to normal. One mechanic told me that there is vacuum leak in the brake system.What you guys think? Can i drive the car without that i destroy something,or should i get the car to the mechanic right away?

 

[The_Jeff]

You should probably get this checked out.

Don't drive like this, but in your driveway, you can disconnect the hose from the vacuum booster and plug it. (I use a spark plug.

) The vacuum booster is the huge (25 cm diameter) circular thing on the firewall of the driver's side of the car. There should be only one big hose leading to it.

Then with it plugged start and idle the car. Note your brakes will work but A LOT STIFFER like this so you shouldn't drive on public streets.

This is something you should have checked out shortly. You can probably make the drive to the mechanic like this. Just be prepared, if the diaphragm ruptures your idle will go up and your brakes will get firm, an bad combination.