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I have a 2005 Jeep Liberty 3.7L. I’m getting a hissing sound when I press the brake pedal, and sometimes – not always – it feels like you have to put more pressure on the pedal about halfway through a stop. It’s almost as if you have normal power assistance at first, but then the level of assistance drops as the car slows down.
The hissing made me think “vacuum leak”, so my immediate suspect was the brake booster. I’ve tried the factory test for the booster – engine off, pump the pedal until it’s hard, hold pedal down, start the engine, check to see that the pedal drops down. It passes this test, but there’s still the hissing sound with the pedal pressed and the engine running. I had a quick look for any obvious breaks in vacuum lines, but I couldn’t find anything.
Pads and rotors are still near-new, and the brake fluid was flushed by a reputable local shop a couple of weeks ago. Brake fluid level in the reservoir is unchanged since the flush, and there are no brake warning lights showing on the dash.
I’ve had a suggestion that the problem may actually be the master cylinder rather than the booster.
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Dune, that test sucks
Try this, unplug the vacuum line going to the check valve on the booster and plug it off. Now start the vehicle and see if you hear the hissing when you hit the pedal. If you don’t it definitely is a leaking booster.
I would venture to bet that you have a bad brake booster.
[quote=”Dunebasher” post=76755] it feels like you have to put more pressure on the pedal about halfway through a stop. It’s almost as if you have normal power assistance at first, but then the level of assistance drops as the car slows down.
[/quote]That is also a very valuable piece of information, you most likely have a tear in the seal around the input rod causing it to start leaking about halfway through its travel. The manufacturer test really wont find that issue it is looking for a big leak, not a smaller intermediate leak.
OK, disconnected the vacuum feed to the booster check valve, plugged the valve opening, and ran the engine. No hiss when the pedal was pressed.
Looks like you were bang on the money, so many thanks for that. If it needed a clincher, I reconnected the vacuum line and held the pedal down so that it hissed continuously, and then shifted through the gears. The hiss sound changed slightly but noticeably as the engine changed idle speed slightly between P and D and then back to P again.
A new brake booster is in my near future, so one more question if I may: seeing as I have to disconnect and remove the master cylinder to get the booster out, and the master is the factory original with 124,000 miles on it, would I be crazy not to put a new master cylinder in there as well?
[quote=”Dunebasher” post=76857]OK, disconnected the vacuum feed to the booster check valve, plugged the valve opening, and ran the engine. No hiss when the pedal was pressed.
Looks like you were bang on the money, so many thanks for that. If it needed a clincher, I reconnected the vacuum line and held the pedal down so that it hissed continuously, and then shifted through the gears. The hiss sound changed slightly but noticeably as the engine changed idle speed slightly between P and D and then back to P again.
A new brake booster is in my near future, so one more question if I may: seeing as I have to disconnect and remove the master cylinder to get the booster out, and the master is the factory original with 124,000 miles on it, would I be crazy not to put a new master cylinder in there as well?[/quote]
Personally, I would not. And you can normally move the master without taking the hydraulic lines off if you are careful not to kink the lines.
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