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There's a pump that runs when the car is on to pressurize the hydraulics, right?

When bleeding the brakes, should the car be on or off so this pump is on or off?

I'm specifically talking about a Honda '03 if that affects the question.

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Are you asking about bleeding the brakes at the wheels to stiffen the system? Or about a system flush or ABS module replacement? My answer addresses the former, Larry's addresses the latter. –  Ehryk Aug 25 '12 at 19:55
    
I was talking about to get air out of the system, not for a full flush. I did see elsewhere that you should activate the ABS to get the fluid out of there if you're doing a flush. I picked the answer I did because for some reason the explanation gave me more confidence I should be bleeding them with the pump off to get the air out. Thanks to both of you for your answers. –  RLZaleski Aug 26 '12 at 18:22
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2 Answers 2

up vote 4 down vote accepted

If what you meant was bleeding the brakes at the calipers to remove air from the system, you should bleed the brakes with the car off. While 'pump' was the wrong word to use, the brake booster runs off the engine vacuum (it's a large diaphragm that multiplies brake force), and this should not be active. You can feel the difference by pumping the brakes a few times with the car off, then pushing with a medium effort on the brake pedal while starting; once the engine starts, the same effort will sink the pedal further and put more force on the hydraulics.

When you bleed brakes, you want to have an assistant push the brake pedal while you crack open the line and then close it BEFORE the pedal hits the floor (or you'll introduce air into the caliper). If the engine is running, not only is this unsafe (front axles can spin, hitting your hands if the front is in the air or moving off the stands if the rear is up), but it drastically changes the pressure at which the fluid will shoot out of the calipers and shrink the time to close the bleeder screw before the pedal hits the floor.

Summary: Bleed brakes (at the calipers) with the engine off.

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The only pump that runs would be for the ABS system. If you are trying to bleed that system (ABS) you would need to make the pump run, if you are just bleeding the base brakes you would not need the pump to run.

For example you changed a caliper and you need to bleed the brakes then no need to make the pump run. If however you were changing the ABS modulator you would need to make the pump run as well as take some additional steps in the bleeding procedure.

For clarification based on @Ehryk's comment.

Honda uses an ABS motor to pressurize the accumulator in the ABS system. Factory bleeding procedures for the ABS system and for brake fluid replacement both require the motor to be run at specific points in the bleeding procedure. The can be done by starting the engine and letting it run for a minute or so, or by hooking up a computer to the car and commanding the motor to run.

Here is a link to a video on Youtube explaining the procedure.

More info here although it's for an earlier model car. I think the 2003 has the motor mounted directly to the modulator assembly.

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You're right technically, that no pump runs for the brakes, but your answer is clearly misleading. The brake booster runs off engine vacuum and makes a large difference when bleeding brakes. –  Ehryk Aug 25 '12 at 10:20
    
@Ehryk I am not refering to the brake booster. I am talking about the ABS pump motor. The vacuum booster doesn't "run" as the OP stated. –  Larry Aug 25 '12 at 19:12
    
@Ehryk I have updated my answer with additional information explaining the ABS pump motor –  Larry Aug 25 '12 at 19:50
    
Thank you for clarification. You can see that the video is about brake fluid replacement; and though this could have been what the OP meant, I'd say 'bleeding brakes' in the colloquial sense refers to caliper-level bleeding ~95% of the time. Someone who replaced their own ABS module and/or was flushing the entire system probably would not be asking how to do it on SE. –  Ehryk Aug 25 '12 at 19:53
    
@Ehryk Honda recommends brake fluid replacement as part of their scheduled maintenance. This is accomplished by bleeding the brakes and includes bleeding the accumulator. Since the OP asked about a pump I assumed that was what he was doing. –  Larry Aug 25 '12 at 20:07
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