hard brake pedal but only for 20 minutes

johnohMay 16, 2006

Car is a 1995 Ford Contour 2.0l

In the morning my brakes act like there is no vacuum at all - the pedal is rock hard. It also idles very roughly and stalls at first.

Then after 20 minutes or so the brakes are normal and the idling is better (though still a little rough). I have searched for vacuum leaks extensively, have replaced the egr, iac, and pcv valves, replaced plugs, wires, air filter, and cleaned maf.

The problem is worst on cold days, which makes me wonder if heat helps some rubber based vac leaks sort of seal up. No idea though.

Any thoughts?

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jemdandy

This problem may require the services of an expert. Erratic brakes and a stumbling engine are ingredients that can lead to an accident. I'm no expert in this arena, but here are my thoughts. On startup, you are missing the vacuum boost to the brake system, and somehow, later on, some or all of the boost is regained. The engine stunbles consistent with a vaccum leak. You've checked for vacuum leaks, but may have missed the "big" one, the vacuum boost unit itself, or the vacuum accumulator and associated vacuum lines. At this juncture, I suspect that the vacuum boost unit of the power brake assembly has failed, and admits an overload of air to the intake manifold (when the brake pedal is depressed) causing the fuel mixture to lean out.

Often associated with the vacuum booster is a small vacuum tank called the 'accumulator'. It will be attached to the vaccum lines that supply the vacuum booster. It's purpose is to have at ready a goodly supply of vacuum volume to handle a suddent demand, and to keep the heater controls happy during this step demand. This tank is very important if the the vacuum fitting on the manifold is 'ported', that is, has a orifice within it to restrict the amount of air flow in case of total vacuum system failure. This tank may have a one way valve. This is the vaccum source that aids one more application of the brake after the engine dies. This tank may be made of two plastic halves and ultrasonically welded together. A leak along this seam is possible.

Do you have a hand pumped vacuum tester with vacuum gauge? If you do, remove the vacuum line that feeds the vacuum booster and apply vacuum directly to the booster. Pump down to a vacuum level, say, 10 psi, and hold. Can the booster hold vacuum or does it leak down fast. If it can not hold vacuum, it is leaking. You may have to block the brake pedal down past the valving point for this test. There is a valve in the booster unit that applies vacuum when the brake pedal is depressed, and it relieves vacuum when the brake pedal is let up. This valve can be a problem source.

    Bookmark   May 17, 2006 at 1:15AM
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john_g

jem's thinking along the right lines. The first check with this complaint is simply find out what the engine manifold vacuum reading is while the car is displaying the hard brake pedal. An engine that is struggling to idle will have very low manifold vacuum, and therefore you will get no power assist for your brakes. The fact that after some period of time it works better strongly suggests that the engine starts running better at idle. Testing would determine what changes over that period of time, and that would lead to a diagnosis and repair. The most likely suspect right off hand is a vacuum leak allowing air to enter the engine that is not being measured by the MAF (Mass Air Flow) sensor. A faulty MAF sensor would also potentially have a significant effect on idle quality, and therefore manifold vacuum. You also could be dealing with a camshaft timing issue, but that would be very prone to setting a trouble code, but not always.

It is unlikely the booster itself is the problem, because if it was you would hear the hiss of air bleeding through it constantly. However, a bad vacuum check vlave or restricted filter between the booster and the engine could be a possible source of this trouble. All in all, it's an easy diagnosis and repair for an experienced tech.

    Bookmark   May 17, 2006 at 8:25AM
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homer_simpson

Why not just start the car and remove the vacum line from the booster to see if it changes engine Idle speed..If it dosnt then its your booster..It might be another censor or the map censor...Rev the engine up a bit and see if the pedal gets better in the morning..Most likely it isn't your booster if it just does it in when starting the vehicle in the morning...

    Bookmark   May 19, 2006 at 12:08AM
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