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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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.
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.
According to AutoZone(AllData) and wherever EBSCOHost gets it's repair data:
Repair Guides for the following vehicles were referenced as they were available:
2003 Honda Civic
1995 - 2000 CRV/Ody
Honda Accord 2007-2008
Brake System Bleeding
NOTE: - Do not reuse the drained fluid.
Always use Honda DOT 3 Brake Fluid from an unopened container. Non-Honda brake fluid can cause corrosion and shorten the life of the system.
Do not allow dirt or other foreign matter to contaminate the brake fluid.
Do not spill brake fluid on the vehicle, it may damage the paint; if brake fluid does contact the paint, wash it off immediately with water.
The reservoir on the master cylinder must be at the MAX (upper) level mark at the start of the bleeding procedure and checked after bleeding each brake caliper. Add fluid as required.
Make sure the brake fluid level in the reservoir is at the MAX (upper) level line (A).
Slide a piece of clear plastic hose over the first bleed screw, and submerge the other end in a container of new brake fluid.
Have someone slowly pump the brake pedal several times, then apply steady pressure.
Starting at the left-front, loosen the brake bleed screw to allow air to escape from the system. Then tighten the bleed screw securely.
Repeat the procedure for each wheel in the sequence below following until air bubbles no longer appear in the fluid.
Refill the master cylinder reservoir to the MAX (upper) level line.
Front: 7 ft/lbs
Rear: 5 ft/lbs
There is no mention that the engine needs to be running. AutoZone's repair guide for the CRV stated that ABS and Non-ABS Vehicles had the same bleeding procedure.
I would still urge you to acquire a Factory Service Manual to ensure that the procedure can safely be performed with the engine OFF, though I suspect that the information available is providing a decent level of certianity if you wish to continue without Manufacturer confirmation.
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.
First of all, when bleeding brakes its always good to look at each car manufacture's recommendation. As far as having the car running, that's only needed if you are doing a complete flush after changing out a failed ABS modulator. Afterwards, you would still need to do a bleeding process with engine OFF as well. If you have someone Pumping up the pedal, make sure the pedal does not go all the way to the floor. I've seen professionals jump up and down very hard while "pumping" up the pedal which is WRONG. Apply a slow pressure when pumping up the pedal. I use a cut off piece of 2x4 underneath pedal to ensure the pedal doesn't hit the floor. Hope this helps.