Strange Brake Behavior

[jklaspok]

I know this is not a 987 forum, but due to the similarities and the fact this forum has some very knowledgeable Porsche people; I thought I would ask here.

I own a 2006 Boxster S where the brake booster works well except when I push in the clutch and tap the brakes as I come to a slow stop. After a few taps it becomes hard as if the booster is no longer operating. Under all other conditions the booster works as it's supposed to. Has anyone had or seen this problem? If so, how was it rectified?

Thanks

[AZ986S]

I don't think the clutch and brakes are related, but to me this sounds like a vacuum leak to your brake booster, or booster is leaking. Had the same issue in another vehicle.

Higher RPM replenishes the vacuum level, but use + low RPM and vacuum is gone.

[paulofto]

The brakes and clutch share the same hydraulic fluid from the reservoir. Check your reservoir level and make sure you have enough in there.

[cheese]

Similar symptoms on my 2005 Cayenne. The fix is to replace the booster vacuum hose/tube/pipe


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[algiorda]

If he is pushing in the clutch, then vacuum isn't dropping, it is increasing cause he is coasting at this point. So this is not a low vacuum issue.

[jklaspok]

I've performed these 2 tests with success:

Test if the Vacuum Hose is the Problem

If the brake booster is not working, it might be the vacuum hose. Here is a simple test to check the health and integrity of the vacuum hose.

1. Park the car.
2. Turn off the engine.
3. Pump the brakes five to six times using no more pressure than you would usually apply when braking. This action allows more air into the brake booster and bleeds any residual vacuum.
4. Now, start the engine, and keep your foot pushing down on the pedal. Note what happens to the pedal.

With the engine running, the engine sucks the air out of the brake booster, restoring the vacuum. Ideally, the pedal should fall towards the floor with you applying light and steady pressure. If the pedal pushes back against your foot, then there is a problem. This could be a blocked vacuum hose, a leak in the vacuum hose, or a leak in the brake booster check valve.

Test if the Brake Booster Can Maintain a Vacuum

The next test checks the integrity of the brake booster. If the booster cannot maintain a vacuum after you turn off the engine, the problem is the brake booster itself, or the failure of the brake booster check valve. To perform this test, do the following:

1. Park the car.
2. Take your foot off the brake pedal and start the engine.
3. Let the engine idle for two minutes.
4. Just before you turn off the engine, push down on the brake pedal and hold it.
5. Turn the engine off and continue holding your foot on the pedal for another 30 seconds.

If the brake pedal stays where it is, then the brake booster can maintain a vacuum. If the pedal starts to rise, it could be time to replace the power brake booster. To be sure, perform the test below to test the brake booster check valve.

[Gelbster]

Agreed but hard brake pedal usually means vacuum loss. Easy enough to test this with a vacuum gauge?

[jklaspok]

You are correct about the vacuum loss. In the near future I'll check for leaks or any other oddities, and let you know what I find. Busy times right now. Thanks!

[jimk04]

Remember the 987 have a mechanical pump for the vacuum. It's in the cylinder head scavenge pump.
In the UK they corrode but not sure of your weather there!

[jklaspok]

I found the vacuum leak. The vacuum tube connecting to the tandem pump had a severe leak. There is a nipple on the corner piece of the tube that provides vacuum to some other component - fuel system maybe? The nipple had broken (I believe before I bought it - used) and resulted in the symptoms posted earlier. It now brakes better than since I've owned the car, and responds well when pumping the brakes.

Whats interesting is the car seems more responsive when I accelerate. That is why I suspect the the tandem pump also provides vacuum to the fuel system. I've read where vacuum issues can result in a richer mixture and degrade overall performance. I've also read it could influence oil consumption. Currently I'm averaging 18.4 mpg and burn a quart of oil every 1200 miles. Any thoughts on my suspicion?

[algiorda]

A photo would be very helpful.