Brake Question

1989 Comanche, 4.0, Aw4, 97k miles.

I noticed this morning that when at a stop light, I hear a hissing noise from under the dash when I have my foot on the brake. When I take my foot off of the brake pedal it stops. It is not real loud, but noticeable when the radio is not on.

No stopping issues, the brakes seem to work fine right now...............I am not really in the mood for surprises, so if this is an indicator of something going south, I would like to take care of it sooner rather than later.

Brake booster?

Mine makes the same noise and it drives me nuts, ever since I replaced the booster with one from Autozone it's made that sound. I have had it in there for about 3 years now with no problems except annoyance.

Like Wagonman I had a hiss for years in my 87, that never really seemed to hurt anything.
If you have cruise in yours it's possible the vacuum switch (down on the brake pedal) is leaking and that's the hiss your hearing.

The way the booster works isn't actually by vacuum. You get vacuum on one side of the diaphragm and the atmospheric pressure (app. 14 PSI) on the other side of the diaphragm helps you when you push the pedal. 14 PSI doesn't sound like much, but will raise one corner of your XJ, about 3-4 inches when pumped in a tire, so there is more work being done than you'd think. It takes a fairly large vacuum leak to affect the process much. The tap for the booster vacuum is right in the middle of the manifold, so there isn't much chance of the vacuum leak affecting just a couple of cylinders (which can be bad for mileage). If the leak gets bad enough it will likely raise your idle and make pushing the brakes significantly harder.
Not likely a panic situation, one of those things I shop around for and replace when the to do list is fairy short.
I swapped my 88 booster with a later model dual diaphragm booster and master cylinder about five years ago, seemed to increase braking significantly, especially in those 80 MPH panic type situations. I lost the write up and link in a hard drive crash, maybe someone still has the link.

8Mud said:

Like Wagonman I had a hiss for years in my 87, that never really seemed to hurt anything.
If you have cruise in yours it's possible the vacuum switch (down on the brake pedal) is leaking and that's the hiss your hearing.

Click to expand...

I did have cruise on the truck when I got it, but I pulled it off because it didn't work. I didn't remove anything from under the dash though. I guess I will try that. Any idea what it looks like?

Thanks!

It's down by the brake light switch, white (if I remember correctly), with a couple of vacuum lines running to it, I'll see if I have a picture or drawing anyplace.