Brake booster affecting idle

1990 XJ 4.0.When I push the brake pedal my idle shoots up like 100rpms. This concerns me because when it does it almost makes an eradic idle and it causes the tranny to jerk just a little bit when I shift into a forward or reverse gear. Does this mean I have a failing booster or is it a normal occurance. I know the booster is tied into the vacuum system but it still doesn't seem normal to me. Anyone know? Help is much appreciated.
-Collin

Does this mean I have a failing booster?
Yes or you have a vacuum leak. Test your vacuum systems first for leaks.

I have a related question, or so I think.
Does the XJ brake booster have a check valve in it?

87manche said:

I have a related question, or so I think.
Does the XJ brake booster have a check valve in it?

Click to expand...

i believe thats what the little black fitting that the vacuum hose connects to is.

If your rpm's jump, your booster has a vacuum leak.

old_man said:

If your rpm's jump, your booster has a vacuum leak.

Click to expand...

Agree. If it was a vacuum leak from the rubber hose that goes from the booster to the intake manifold, the vacuum leak would be present all the time so the idle would always be higher than normal.
Sounds like you'll need to replace the booster so this'll be a good opportunity to upgrade to the dual diaphragm unit found in newer XJ's. If you do this, make sure you also get the master cylinder and proportioning valve to go with it.