Nicole

Remember that there is also suction bleeding. At the factory, they first suck out all air of the system, then let the fluid in. A local, Porsche-trained mechanic (Marvin Weltz at the RennShop in Santa Clara) says he always uses a type of suction bleeding.

One of the advantages of suction bleeding could be that suction bleeding expands the air bubbles, while pressure bleeding compresses them. But since the bleeder screw will not seal when loosened, you will always see some air coming through the suction system - regardless of whether there was air in the brake system before. It may be hard to tell when you got the air out of the system this way.

SharkSkin

I went and looked at the site and video at the link posted by Bill. I cracked up when the hawker started pumping air bubbles into the brake caliper. The thing that concerns me about this approach is that usually, you don't want the crap that is in the calipers to be pushed back up into the master cylinder. Not a problem on Nicole's car at this point, it's just worth noting that sometimes it's the wrong way to do it.

Where I think this would really help is in large cavities like where the springs are shown above the shuttle valve. Normally, it's very easy to move bubbles downhill, through loops, etc in tubing because the surface tension relative to the I.D. of the tube ensures that while the fluid is moving, so are the bubbles. In a large cavity I would expect the bubbles to just collect, and here the reverse bleeder may be just the thing to get them out.

What my tool will address is moving the shuttle valves up and down rapidly while running the pump(and various combinations of either/or). This may dislodge bubbles and/or break them up so they can mix in the flow produced by the power bleeder. v1.0 did get some bubbles out after what, 3 or 4 quarts having been run through the system prior to my attempt? The pedal did firm up. We quit when we let some of the smoke out of one of the relays. I've addressed that problem in v1.1 and I have replacement relays to put in. We should be able to run it more or less continuously at 50% duty cycle.

One thing that concerns me is the accumulator shown in the schematics between the shuttle valve and the pump. If it really is oriented that way and it has air in it, I don't see any way to deal with it other than a vacuum pump, some drilled bleeders, etc. etc.

Ron_H

Quickly....has the problem been solved yet?

SharkSkin

Ron, if you mean the problem with Nicole's car, no, but we did get some improvement. I edited my post above to make it clear that we did not pump several quarts through the system when I was last there, but that a LOT of fluid had previously been pumped through during various attempts to resolve the issue. We hope to make significant progress today. If you mean the problems with my gizmo -- I think so, but I need to get out in the garage and finish up, then run one final test. I have a couple of ABS units here I can test with before I hook up to Nicole's car again. Note that this tool completely isolates the ABS unit from the car's electrical system other than drawing 12V so it's pretty safe.

As Bill says in post #14 it's really not an issue(or shouldn't be) if some basic precautions are observed when changing calipers/hoses. For example, if you're going to be replacing the hoses anyway, pinch them off with vise grips before disconnecting the caliper(don't do this on hoses you intend to re-use). Then, with the fresh calipers installed, quickly replace each hose and fill the caliper(at least the inner half) in turn with the power bleeder.

Also, for other readers, on non-ABS cars it's 100% not an issue. I have drained my whole system and replaced the MC without bench bleeding, and was able to get back to a rock-hard pedal with absolutely zero drama using the Motive power bleeder. With ABS the key is to not let air into the ABS unit in the first place, and eventually we hope to have a clear solution for dealing with that issue should it arise.

Ron_H

Thanks Dave. Very informative.

SharkSkin

OK, it checks out. Letting the soldering iron cool so I can pack it(just in case) with the other stuff I'm taking to Nicole's, then I'm on the road.

Mrmerlin

Good luck, I hope this works, so Nicole will feel comfortable driving her car again...

SharkSkin

Well, we now know of two more methods that won't solve this problem.

My tool worked as intended, it cycled the solenoids and the pump, but that just was not what was needed to solve the problem.

Bill's reverse-bleeder also worked as advertised, but that also was not what was needed to solve the problem.

Nicole is going to find out what she can from her contacts in Germany about how the system was filled during production. I think it should be easy enough to draw a hard vacuum on the hydraulic side of the system and let the fluid get sucked in, as was done at the factory. We don't know which end the vacuum was applied at, or which end the fluid was introduced at but Bill and I tossed some ideas back and forth.

A reservoir is needed, since fluid will come out of the lines and possibly out of the calipers; we don't want this getting into the vac pump. Also, the pistons may retract under vacuum so we need to be able to capture that much fluid. I'm thinking that an adapter to connect a Motive bleeder directly to the MC with a shutoff valve(or two, one per circuit), and some drilled out bleeder nipples with valves silver-soldered to each at the caliper/vac end would facilitate this, along with a manifold to apply vacuum at all 4 calipers at once.

The tool I built was not a complete waste of time, it can be used to test basic functionality of the ABS pump and solenoids on the car.

Anyone need some spare ABS pumps and relays?

Ron_H

I suggested to Nicole a source of information to solve this problem and this source is the best I've ever known for Porsche. What I can't figure out is what she has to lose by accessing it?

AO

Can you guys recap what all has been done/replaced on Nicole's brake system?

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

Nicole

Summary:

- Front calipers rebuilt with new seals
- Rear calipers rebuilt with Porsche/Brembo kits (Piston/seals)
- All flexible brake lines replaced
- Master Brake Cylinder replaced
- Bled more than 18 times, including PowerBleed, PedalPush, Reverse, and with ABS activation during bleeding
- Brakes respond and even trigger ABS, but pedal travel is almost 2" longer, and pressure point is soft

It all started with a broken off bleeder nipple on one of the front calipers...

I did not want to risk having worked on the caliper when I had no other one to replace it with. So I bought a used pair of calipers from a fellow rennlister. Together with a friend, we replaced the piston and dust seals from Zeckenhausen Racing.

I planned to repaint both the new front and old back calipers, and replace all the flexible brake lines while they were off.

When we removed the front calipers, we were surprised that no brake fluid dripped out, so we let the front lines hang, while we plugged the rear lines. This was probably a big mistake, but we did not realize it then.

I refinished the front calipers, and also had one or the rear ones rebuilt, but decided not to refinish the rear ones (they were not as bad as I had thought, and cleaned-up nicely.

Then, the calipers were mounted back on the car, with the new brake lines, and the system was bled with the PowerBleeder.

The pedal feel was spongy, and the pressure point near the floor.

We bled a few more times with PowerBleeder and Pedal Pushing, and went on a test drive, hoping to dislodge the remaining bubbles. During this exercise we overheated the rear brakes - which is near impossible on a 928, as I am told.

The system was bled a few more times, and the symptoms described here as well as via phone to Tech Support Master Mr. Merlin - who suspected a damaged Master Cylinder.

I ordered a new master from Roger, who expressed doubt that this was the problem. However, I wanted this resolved and pressed on. The new master was installed - and made ZERO difference after the bleeding.

I took the car to the local Porsche dealer, who bled it twice and triggered the ABS a few times, and declared the brakes functional. Well, yes, they did brake, but the pedal travel was still 2 inches longer, and the pressure point softer than it was before.

So, we kept trying to bleed with various tricks (tapping calipers, triggering ABS between bleeds, etc), with no improvements.

Then Dave offered to build a gizmo that can control the ABS pump and valves without frying any electronics. On the first try we did get some bubbles out, even though the gizmo did not work perfectly, yet. We achieved an approximate 10% improvement of the pedal feel.

Today, Dave brought his re-engineered gizmo, which did a great job in controlling the ABS pump and cycling the valves. We did a combo of Power Bleeding, Pedal Pushing, and running the gizmo, but no bubbles, and no improvement.

Then Bill tried reverse bleeding, with generous amounts of fluid - again no improvement.

When I test drove the car tonight, it almost immediately started burping in the system as I had never experienced it before. It was pretty loud and vibrated the brake pedal, but did not change its feel. This happened only once, and did not happen again after subsequent starts.

So, here I am with a 928 that's been sitting for almost 9 months with spongy brakes (no, it's not pregnant with mini-sharks). I am not comfortable driving it that way, because it takes much longer than usual to get the brakes to respond, and even then they don't inspire much confidence. Will they be able to stop the car from 120mph, if needed? Or going downhill in the mountains? Or will the bubbles start some boiling action and let me lose the brakes?

Yes, Ron's contact is on my list to try, and I will also work my contacts in Stuttgart.

I want to take this opportunity to thank everyone who has looked in to this and helped in finding the solution for this nasty issue!

Nicole

We cannot put the old calipers on because of the broken bleeder nipple. There is no way to bleed that caliper!

Bill tried to get that nipple out, but without success. This is a whole other can of worms...

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.

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 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.
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.