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Discussion Starter #1
i have a 03 da/alli 4x4 ec. Every now and then after i have been driving for about 15-20 min. my brakes will begin to engage on their own. You will first start feeling a vibration in the rear end. It when then begin to slightly vibrate in the front end. If you continue to drive the vibration will get bad enough to rival even the most warped rotors I have ever felt. When it begins to engage the brakes you can begin to feel a noticable lag in power.

The only diagnosis that me and my buddies can come up with is that the Hydraboost unit is leaking some pressure by the valve which in turn is engaging the master cylinder causing the brakes to begin to engage.

Any ideas anyone??
 

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have you had the abs system checked for codes? you could have a faulty abs module causeing the brakes to pulse. i highly doubt its the hydroboost. if it was the hydroboost, youd also lose power steering...p/s pump provides the hydroboost unit with fluid and pressure at a constant rate (as the p/s pump pulley turns)
 

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Someone else here is having your problem ,last I heard they were replacing the hydra boost .
 

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Discussion Starter #4
holtzer you won't lose power steering if the pump is still good. My pump is still good I just think that the Hydraboost is not completely returning back to close the valve.
 

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Below is a step by step procedure for diagnosing your problem. There are several scenarios which leave the brakes not working correctly and the power steering working just fine. The pump supplies fluid to both units, but they each have a choice as to what they do with it.

Run pump at fast idle.

Pull the brake pedal rearward with approximately 10 lbs. force and release. Measure the distance to the floorboard.

Make 100-lb. brake application. Release brake pedal and measure distance to floorboard. The brake pedal should return to its normal position (step 2). If measurements are the same, go to step 5.

If the brake pedal does not return properly, check the pedal to be sure it is operating freely. Correct any sticking or binding as necessary.

If brakes are self applying and pedal is free, check for obstruction in the return line or a kinked connection between hydo-boost and pump reservoir. If obstruction or kink is found, go to step 6, otherwise go to step 7.

Remove obstruction or replace line as required. If condition remains, check for a damaged reaction end. If damaged the hydo-boost should be replaced or repaired.

If brake pedal is free of any binding conditions and return line is free of obstructions, remove the master cylinder cover.
Observe the brake fluid in the reservoir while rapidly depressing the brake pedal one inch.

Fluid surface should have some movement or spout in the forward reservoir section. Minor spouting may occur in the rearward reservoir. If no movement or spouting of fluid occurs in forward reservoir, the hydro-boost unit is defective and must be replaced or repaired.
 

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Discussion Starter #7
Update on my brake problem. Replaced the Hydra-Boost unit and now the brakes are working right. The only issue now is that the booster that I got is leaking from the front seal. So it looks like that I get to replace it again. The place that i got it from is sending me another one it should be here Tues.
 
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