StratfordShark

Why is it recommended to try removing abs relay, driving, then re-bleeding in case of a soft brake pedal?

the flyin' scotsman

Which ABS relay............the one on the fuse/relays panel?

I'm not sure I've heard of this process but replacing all 3 ABS relays cured my ABS/braking issue.

Mrmerlin

are you having a problem?? or just doing some research??

StratfordShark

Hi MrMerlin,

It's a long(ish) story.

I do have a pedal which feels soft. Have bled a couple of times with Power Bleeder but the brakes still don't have the firm/sharp feel I'd like (and which I've experienced on another S4). The brakes do pass the local regulatory tests though, albeit I have slight binding on one front brake (I know the sideplates are lifting as I've had to grind pads to fit them).

I think the problem goes back about a year to when I changed all pads and rotors. At the time I would pump the pedal after fitting the new pads (having retracted the pistons) so as to bring the new pads back in contact with rotors. I'm paranoid that maybe when I pumped the pedal I caused corrosion in MC to damage a seal. Maybe not though as the MC is only 4 years old and the pedal firms up fine after a few pumps when engine is off.

When I was looking through another thread the other day I saw a post by suggesting the ABS fuse/relay pull prior to bleeding as a possible fix for original poster's soft pedal feel. Another poster said that the ABS should be on (ignition in run position) to allow relay to kick in when you bleed barkes the old 'pump pedal' method. That poster was dubious that a Power Bleeder could remove air bubbles fully.

I'm planning ab ig brake overhaul anyway with new calipers (not much more expensive than trying to refurb the sideplates) and brake hoses. As this is in my 928 future I wanted to explore all other avenues for effective bleeding too so that I could be more certain of h aving decent brakes when the project is all done. So yes I have a problem (to my mind anyway - the brakes don't inspire confidence), but yes I am also researching at this stage!

mark kibort

dont use the power bleeder, as it can leave the pedal very soft. I dont know why , but ive experienced this before. use the traditional method of bleeding by pushing the pedal down with the bleeder open, close it and let the pedal up. (two man method). removing the abs relay does nothing but eliminate the ABS functionality

Mrmerlin

though the way MK suggests will work OTHER damage can occur. This is usually from a corroded master bore, during normal function the master piston only travels about an inch , when the pressure is bled off and the master is pushed all the way down then the seals will begin to travel over unused portions of the master bore.
This is fine if you have a relatively new master and have done fluid changes on time, otherwise unused portions of the bore can begin to rust and thus the bore will have portions that may damage the seals thus causing the master to fail.. If you have doubts about your master then you can try to open it up and check the bore or replace the master

the flyin' scotsman

I've used the power bleeder many times and it works fine.

From bleeding completely empty calipers, to changing fluid and swapping out the OEM lines for new SS lines it has worked perfectly.

UKKid35

All you need is a bit of brake fade, and then you find yourself using the entire bore very quickly...

I was concerned about this issue myself because I'm certain my brake master is original, however I've not had any problems despite using the non-power bleeder method several times.

no doubt

Yup. You want to hook up the simple Motive power bleeder to the reservoir, plus you want your ignition On, plus you want to physically pump the brake pedal enough times on each corner bleed to kick on the ABS pump. All 3 things should be simultaneous.

In this manner you can do a 1-man full (all new fluid) brake bleed and still get a firm brake pedal. Otherwise, just do the manual 2-man brake bleed.

worf928

Upon what do you base this recommendation? The WSM does not indicate any requirement other than hooking up a power bleeder and bleeding. It does not indicate that the ignition has to be on for ABS-reasons (or any other reason.) (I'm not suggesting that for other - newer - cars that this is the case, just for 928s with their early-gen ABS.)

no doubt

NEWSFLASH! ABS pumps hold brake fluid. Old brake fluid. When doing a full brake bleed to change out the old fluid, you want to also change the fluid inside the ABS pump.

This is most easily accomplished with the ABS pump turned on and pumping.

Likewise, even doing a partial brake bleed will be difficult on the front brakes if the ABS pump isn't active. Do you really want to try pumping fluid through the ABS pump with the ABS pump OFF?!

So I see no reason to leave the ABS pump inoperative during a brake bleed.

Turn on the ignition. You can leave the motor off, but the ignition should be on and you should manually press the brake pedal enough times to kick on the ABS pump.

the flyin' scotsman

What?..............I should add that I changed the proportioning valve and bled the system with the power bleeder with excellent results.

Pedal is rock hard and brakes well beyond the power of the car.

Jim M.

+1

+1

Lastly, the ABS module is NOT a pump, it does NOT increase the brake pressure. It senses wheel lock and RELEASES pressure to the locked wheel. You can never have more pressure than that produced by the master cylinder with your foot on the brake.

worf928

Please comment on WSM 47-23 "Note: There is no difference in bleeding procedures for cars with ABS."

If there was any reason for the procedure to differ from a non-ABS 928, I would think that the factory shop manual would state something other than the exact opposite.

I've power bled and two-man non-power bled, dozens of 928s with ABS. The only time I had a issue was when pads, rotors, and lines were simultaneously changed. It took a bit of doing to get all the air out in that case. And that had nothing to do with ABS.

Since, by all appearances, the ABS operates (or not) based upon input from the four wheel speed sensors, how will pressing the brake pedal, on a stationary car the brakes of which you are bleeding, cause the ABS pump to do anything at all?

Does the ABS pump have a hydraulic pressure sensor in it? How else would it be connected to the brake pedal and "know" that you've pressed the pedal?

I know that a lot of newer cars with more-sophisticated ABS system must have the ABS system activated in a special mode for proper flushing. But, there does not appear to be any reason to suggest that is true for the 928.

Now, I didn't design the ABS system in 928s. Maybe you did. If so, speak up.

no doubt

Turn on your 1989, 1991, or 1994 928. Hold your foot on your brake while starting the motor and for the next 30 seconds.

Notice that you can feel your brake pedal buzz at least once during that time (if not, you may have to put your car into gear for this test to occur). That should be your ABS pump pressurizing up and testing your brake system.

Consider that brake fluid passed through that ABS pump for said event.

How do you propose to get all of the air out of your brake system, or to replace all of your old brake fluid, if the fluid inside that ABS pump is held by an ABS pump that is off (of course, some fluid *can* pass through said pump...that's a safety feature in case of ABS pump failure)?