Hissing while Braking?

Hey Guys,

My '89 XJ hisses when i step on the brakes. Basically, it sounds like someone is exhaling when i step on the whoa pedal. Sounds like its coming from the area directly behind the firewall.

Could anyone give me some ideas on what to check, or what could be the culprit? Brake booster? I don't see any brake fluid leaking. I'm fairly new to the XJ world. I've already bled the brakes and it has helped bring the miserable braking performance to an decent level, but it still pales in comparison with my other cars (toyota and honda cars).

Any advice would be most appreciated! Thanks!

Alan

you might want to check the booster for vacuem pressure...

Here is how you check a power brake booster for proper function:

1. With the engine OFF, pump the brakes repeatedly. The pedal will get progressively harder. Once it gets no harder to pump, HOLD THE PEDAL DOWN firmly.
2. Start the engine while HOLDING pressure on the brake pedal.
3. If the power booster is functioning normally, the brake pedal will move further down as the engine starts, and manifold vacuum builds.

If the pedal does not behave as indicated in step 3, it's time for a new booster (they are not rebuildable.) The cheapest source would be a junkyard.

AZ Jeff said:

Here is how you check a power brake booster for proper function:

1. With the engine OFF, pump the brakes repeatedly. The pedal will get progressively harder. Once it gets no harder to pump, HOLD THE PEDAL DOWN firmly.
2. Start the engine while HOLDING pressure on the brake pedal.
3. If the power booster is functioning normally, the brake pedal will move further down as the engine starts, and manifold vacuum builds.

If the pedal does not behave as indicated in step 3, it's time for a new booster (they are not rebuildable.) The cheapest source would be a junkyard.

Click to expand...

And should it be bad, you should just do the upgrade to the better power booster. 95 and up double diaphram style...

Actually you could use the later dual diaphragm booster from a 1995 or 1996 year ONLY. You need the 1/4 inch aluminum spacer that goes between the booster and fire wall too. Other years the rod to the brake pedal is the wrong length. You also need the 1995 or later (any year after 1995) master cylinder, which is shorter than the mc used with the single -diapragm booster. Your existing mc is too long and won't fit with the longer dual booster. Then there is the issue of adpating metric to SAE brake lines, and adapting the brake light/auto tranny unlock/cruise control switch to work with the different shaped new booster rod. For the full info go to www.madxj.com, then select "technical", and read the pre-95 brake booster upgrade article.

Chances are all it is is the foam/boot around the booster rod. When the engine vacuum is pulling on one side of the booster diaphram the other side intakes air through that foam to fill the volume left by the movement of the booster diaphram. It's just an annoying noise, not a problem. Usually the boot is ripped you hear that sound from the air rushing through a small hole.

stoneattic said:

Chances are all it is is the foam/boot around the booster rod. When the engine vacuum is pulling on one side of the booster diaphram the other side intakes air through that foam to fill the volume left by the movement of the booster diaphram. It's just an annoying noise, not a problem. Usually the boot is ripped you hear that sound from the air rushing through a small hole.

Click to expand...

Wow, thanks everyone for the suggestions.

I'll be checking the booster for both vacuum and a torn boot around the rod first, although the later model booster conversion sounds tasty.

Out of curiosity, anyone have any luck with playing with different brands of brake pads to help braking performance?

Thanks,

Alan

we install the jeep "valueline" pads. they are competitively priced and backed by chrysler. and we rarely have a squeaking problem.