SF Bay Area 928ers: ABS Bleeding

[Andrew Olson]

Thanks for the long laundry list of what you have done.

I had a spongy feel on my brakes when I swapped calipers. I bled and re-bled and was never very satisfied with the pedal feel.

Let me ask this, when you changed the calipers, did you put new pads on?

I did, and I think this was the cause of my spongy pedal feel. IIRC I had installed Mintex pads. At first I thought it was maybe the pad bedding into the rotor, but it seemed to never get any better.

I decided maybe a more agressive pad might be order. So, on a whim I ordered some Hawk sport pads. I can't remeber if I re-bled/flushed the brake system at the same time (probably at least bled the brakes) but since then the pedal has been firm and the stopping power is awesome! Just something to consider.

[Bill Ball]

Originally Posted by Mrmerlin

Bill I also stopped by Nicoles a few months ago and tested the pedal it is definitely lower than any other S4 I have checked.

That's the primary issue.

That said is it possible to take the broken caliper to a welder and have them weld a nut to the remaining portion of the ezout/bleeder nipple?
Once this part is heated the bleeder should be able to be removed.

I got 99% of the broken bleeder out. Usually then the shell can be picked out, but the remnants seem welded to the caliper and I am damaging the caliper threads.

One other thing have the front brake hard lines been checked for damage( IE crushed) since previously Nicole hit a rock at SATL and may have also damaged other parts??.
If the rear brakes are overheating then the fronts are not working this could point to a damaged MC or line restrictions, is it possible that there are plugs in the lines of the new calipers that may have not been removed??. I think its time to recheck and inspect all of the work that was done as the brakes were working fine B4 the bleeder nipple was damaged.

The rear brakes did that a long time ago before we got a bunch of air out. That issue is gone. There is good action at all 4 corners.

[SharkSkin]

Originally Posted by Bill Ball

I finally studied the ABS system a bit tonight, and I think we could be chasing a red herring. In the resting state, shown below in one of the pictures Dave included earlier, the solenoid valve (c) isolates the ABS return bypass pathway reservoir (g) and pump (h) from the rest of the brake system. Fluid just flows directly from the brake master cylinder to the calipers through a path in the solenoid valve, while the ABS mechanism on the right is blocked off.



So, unless the solenoid valve were bad, leaving the brake system open at the calipers should not allow air to get into the ABS reservoir. If the solenoid valve is bad, then Nicole has another problem. Also, even if there were air in the ABS reservoir and pump cavity, it is isolated from the rest of the brake system except when the ABS is in drop pressure mode. So, a normal brake bleed should clear all the passageways used by the brakes except under ABS conditions.

I'm beginning the believe we are not dealing with air. We did have a lot of air in the lines originally. Once that was removed, the brakes felt fairly normal to me. ABS could be activated, showing that the car had good stopping power, BUT the pedal "play" was excessive: 2-3 inches compared to about 1 in my car. Nothing was done that should require adjustment of the actuator rod. Anyway, I need to think about this some more. I could be wrong in my characterization of Nicole's brake pedal feel.

Bill, my original thinking on this was that air was trapped in the large cavity where the solenoid springs live. I speculated that the air got in there either through the disconnected lines, by not bench-bleeding the new MC before installation, or maybe both. Coming to the party a bit late, I had the order of operations a bit mixed up, having heard the details in fragments -- so letting it sit with the lines open seems more likely. Though above, Nicole says no fluid was lost, so now I'm scratching my head again.

Nicole, are you saying no fluid was lost at any time while the lines were open, or just that you didn't see any fluid initially, but fluid came out later? Also, how long were the lines connected to dry calipers before the first attempt at bleeding? Were the pistons in the calipers pushed back into the retracted position while they were hooked up, or were they already retracted when you connected up the hoses?

Anyway, back to my original thinking -- In a cavity this large, there is no capillary action and no amount of bleeding would remove air from this area. What I was hoping to achieve was pushing air in this area upward into the feed line when the solenoid was activated, past the outlet port on the pump, which would then force fluid and bubbles back up to the MC(This is one of the reasons I had the bleeder pumped up to only 5 psi -- maybe even that was too much). This didn't seem to be happening, since with the solenoid activated and the pump running we did not see the expected reverse movement of fluid at the bleeder. It could be that all it's doing is moving the accumulator diaphragm back and forth, or worst case the pump has lost its prime.

When the first iteration failed, I thought that maybe rattling the solenoid back and forth would break up any bubbles in that area making it easier for them to get caught up in the flow from the power bleeder(leading to the addition of the square-wave generator), but that may have been a fool's hope -- we don't get that much flow and it's still a pretty big cavity -- no good way to force the bubbles through. I think one reason we saw bubbles on the first attempt at activating the ABS pump is because with pressure on the pedal, we saw much faster flow when we cracked the bleed screw. So IMHO breaking up the bubbles plus having a high fluid flow rate had some positive effect -- but not nearly enough.

I had hoped that your reverse bleeder would push any air in that area back up to the MC, but there is still room for lots of bubbles to be trapped at the upper spring seat. We should have also tried doing that with the solenoid held open and the pump running to ensure that that circuit was purged. Too bad we ran out of time.

[SharkSkin]

Originally Posted by Mrmerlin

the only thing i can think of now is to replace the old calipers and try it.
If then it is a no go then replace the already replaced master cylinder, one of these will hold the key to this problem

Stan, replacing the MC had no effect on the problem, so I think it's unlikely that the problem is there. The only way that I could imagine the calipers being at fault is if the seals are sticking, causing it to retract too much. I was watching closely though, and never saw any movement there, and could not wiggle the pads at any point in the process. Am I missing something here? It sounds like you are just working from the principle of "reverse the last change that you made" -- which is valid to a point -- but I figured I'd ask if you think there is something more to it than possible sticky seals.

[LarryM]

Here's an old motorcycle trick that I've used for years. It speeds up bleeding the front calipers a great deal. You have probably already done this but just in case.

Have pressure on the pedal, wait a second or 2, then with a rag and a wrench crack open then shut the line coming out of the master cylinder. 2 person job and usually engine off.

Air loves to rise and on my Ducati's front Brembo system, that long line going up the the handle bar MC was always a pain.

I'm crossing my fingers for you.
Larry Marble

[Bill Ball]

Hi Dave:

OK, I thought you were more concerned about the ABS reservoir.

Anyway, as to the solenoid chamber in the regular brake pathway, I can't see how air would get trapped there for very long. Yes, it looks from the schematic that there is a small recess in which the upper end of the spring sits that might trap some air but it is mostly filled with spring coil. And it would appear that any kind agitation should purge any bubbles. Certainly all the agitation from your ABS activation tool should have cleared that out. I think we need to take a step back and look for other things that could be involved or at least more carefully observe the symptoms to make sure thay are consistent with air in the system and not something else.

[SharkSkin]

Originally Posted by Bill Ball

Hi Dave:

OK, I thought you were more concerned about the ABS reservoir.

Anyway, as to the solenoid chamber in the regular brake pathway, I can't see how air would get trapped there for very long. Yes, it looks from the schematic that there is a small recess in which the upper end of the spring sits that might trap some air but it is mostly filled with spring coil. And it would appear that any kind agitation should purge any bubbles. Certainly all the agitation from your ABS activation tool should have cleared that out. I think we need to take a step back and look for other things that could be involved or at least more carefully observe the symptoms to make sure thay are consistent with air in the system and not something else.

Bill, I think that your reverse bleeder may be the most promising approach to get most of the air out of that cavity. Also, if the pump is not moving fluid(as seems to be the case) there could be air between the check valve and the tee in the fluid path. Also, if there is any debris in that check valve then fluid is flowing back into the pump. Lots of little mysteries. The calipers could be eliminated as a cause if we could get the right size banjo bolt and nut and simply remove the lines from the calipers, then cap and bleed them. If the pedal firms up, the problem is in the calipers. If it does not, the problem is upstream.

[Mrmerlin]

is it possible that the ( front calipers) are for a different application and thus not correctly fitted? Say from a 951 .
Dave thats a great idea to remove the front calipers from the system.
From all of this I would think that returning the system to how it was B4 the new parts were installed you may find the answer.
Still get the old calipers and install them, if the bleeder nipple is broken weld some metal to whats left and this should be enough to remove it, the steel weld wont stick to the aluminum caliper.

[SharkSkin]

Stan, the bleeder that was broken broke under the surface, and with Bill's subsequent attempts to get it out there is nothing to weld to. I suggested having a machine shop modify it to take the next larger bleed screw, or get all of the steel out, weld over the hole and drill/tap a new one. Probably not worth it, given what replacements cost and given the safety implications. That's why I suggested a way to eliminate the existing calipers as a possible cause.

[Nicole]

The front calipers come from a 91 928 GT. Only non-standard item about them are the replacement seals from Zeckenhausen. I have complete BREMBO rebuild kits in the garage, that I intended to use for my old calipers. But we could take the ones that are on the car, and re-rebuild them with the BREMBO kits.

The pedal is not just having much longer travel, the pressure point is much, much softer than it should be. My Saab brakes feel 10 times firmer than the 928 brakes right now.

[Lizard931]

have you gotten a pressure guage yet?

Wilwood maked them and you can thread it in a see how much pressure is able to be built up to see even if the MC is moving the fluid. Compair on another car with good brakes.

[heinrich]

Nicole I will be there on Sunday. Maybe I can assist also?

[heinrich]

Adrew I drove Nicole's car HARD recently and repeatedly .... got all 4 corners so hot you could fry eggs on them. Braking performance was 100% every time but pedal travel was definitely beyond normal tolerances, in other words I bet money there is air in the system.

Originally Posted by Andrew Olson

Thanks for the long laundry list of what you have done.

I had a spongy feel on my brakes when I swapped calipers. I bled and re-bled and was never very satisfied with the pedal feel.

Let me ask this, when you changed the calipers, did you put new pads on?

I did, and I think this was the cause of my spongy pedal feel. IIRC I had installed Mintex pads. At first I thought it was maybe the pad bedding into the rotor, but it seemed to never get any better.

I decided maybe a more agressive pad might be order. So, on a whim I ordered some Hawk sport pads. I can't remeber if I re-bled/flushed the brake system at the same time (probably at least bled the brakes) but since then the pedal has been firm and the stopping power is awesome! Just something to consider.

[Nicole]

Pressure gauge sounds like a good idea - but where exactly do you connect it?

Yes, Heinrich put the thing through its paces. I don't dispute that it brakes. The trouble is the lack of a defined pressure point, and the travel that's way too long. Very often, when I press the brake pedal, I get hung up on the arm that is behind the gas pedal - that's how low the brake pressure point is now!!!

I don't trust this, and would not attempt to drive hard with this condition. But that defeats the whole purpose of the car. I can cruise more comfortably in waSAABi. The 928 is for having fun, but that's not possible safely these days.

[Lizard931]

You could get 2 and put one on the master cylinder (replaces bleeders) and another on a caliper.