Bleeding ABS Pump

[larrybard]

A recent Der Gasser article implies -- unless I am misreading it, which is quite possible with anything even remotely technical -- that without a special (and presumably very expensive) tool, the PST2, the ABS pump cannot be bled.  Which raises a number of questions, the most important of which is, I think: How important is it that the ABS pump be bled?

[Larry Herman]

That is correct. It is done with a tool called the Bosch Hammer, which will cycle the ABS pistons and pump out any air. Otherwise, when bleeding the brakes, the fluid just travels past and any air trapped there will remain.

[larrybard]

Larry,

So -- and maybe you have to dumb this down a bit for me -- anyone who either bleeds their own brakes, or has a mechanic/garage do so -- will in effect be having  deficient (unsafe) brake bleeding?

Larry

[Larry Herman]

It is rare to get air into the ABS system (unless you have opened up the braking system upstream from the unit) and normal bleeding works fine 99% of the time.

Cycling the ABS unit is usually a last resort when trying to correct a soft pedal, or if it is possible that air has gotten into the system from a master cylinder change, line repair, etc.

[michael lang]

As a side note, alot of the techs that work for me will go so far as to disconnect the battery to keep the ABS pump from cycling during the brake bleeding. That usually doesn't happen but I can remember one time when it did. Luckily the tech didn't have his hands anywhere near the caliper when it happened, he could have really hurt his hands.

 

[Terry]

I have always assumed that there is a very small amount of fluid in the ABS pump at rest and when we activate the system on the track, which is pretty common, whatever air is in there is purged and replaced with fluid. That miniscule amount of air will then travel and be bled in due course. My $.02 - Larry, sound right to you?

[Larry Herman]

Terry, I do not have a diagram of the internal workings of the ABS module, but from talking to one guy I know who is familiar with it, the unit has valves in it which cycle rapidly, momentarily relieving the pressure on the brake caliper piston. The normal flow of fluid is right past the "front side" of these valves. Air can become trapped behind these valves, and the regular bleeding process will not purge it. The valves must be cycled as you bleed in order to get the trapped air out.

[larrybard]

Boy, am I even more confused.  (Maybe I'm starting to regret having started all of this.)

Larry, you seem to say that the ABS pump valves should be cycled during the bleeding process -- and Mike seems to say the opposite, i.e., precautions are taken (for safety reasons) in order to prevent cycling during the bleeding.

[Larry Herman]

Just read it all again, it makes perfect sense.  You bleed the brakes normally.  All fluid passes through the ABS unit (and past the closed  ABS valves) no problem.  Imagine that the system is set up like a "T" with the fluid going through the T, and the ABS valve on the branch.  When that valve opens, it dumps pressure to the specific brake line where the wheel is locking.  That releases pressure on the brake caliper and prevents lockup.

Where the "dumped" fluid goes in the ABS is still a mystery to me, but safe to say that is where the trapped air is, and that is why you would need to cycle the ABS unit AS you bleed the fluid through.  Again, the need to bleed tha ABS unit itself is very rare.