heliflyer

Thought I would post this as it may be of help to Nicole.

Have just completed a caliper rebuild on all four corners and ended up with the same problem as Nicole - no matter what I did the brake pedal had a lot of travel on it before applying the brakes. I had put around 4 litres of fluid through the system using a combination of pressure bleeding, vacuum bleeding, speed bleeders - all to no avail.

I finally reverted to the old two man system of brake bleeding, one pumping the pedal then holding the pressure on the pedal while the bleed valve is opened. Close the bleed valve when the pedal is on the floor, then repeat.
I did this starting with the master cylinder, then each caliper, working from closest to master cylinder to furthest away. On each caliper I also started with the outside bleed valve, then did the inner one.

I now have a good firm pedal with only the 10mm movement before the brakes are applied.
I hope this helps Nicole.

Nicole

Thank you so much for posting this and giving me new hope! It's quite frustrating to have a 928 "SUV" sitting in the garage (928 high on jack stands)

We have now bled at least 20 times with little to no improvement - including the procedure described by you. Bill suggested to remove and inspect the Master Cylinder, but while I'm not opposed to it, I'm a bit skeptical that it would yield any findings. After all, the MC was not touched in the initial work, and replacing it with a new one did not make any difference, either.

Your post gives me new hope that it may be some bubbles lodged inside the system. Which is what the pedal feels like.

How often did you pump the pedal per bleeder nipple? Was the car level a the time of bleeding, or was it sitting at an angle?

One thing we have not tried was suggested by a German 928 mechanic: He told me to lift the car in the front as high as possible. while bleeding. Has anybody tried this?

Again, thank you very much for thinking of me and posting this!

heliflyer

Hi Nicole,
Pedal was pumped 6 or 7 times for each bleed nipple and pressure kept on the pedal as the bleed nipple was opened, allowing the pedal to sink to the floor. Bleed nipple closed before allowing pedal to rise again.
Car was pretty much level on jackstands at the time of bleeding.

Nicole

Did you push the pedal all the way to the firewall, or stop shortly before it to avoid damaging any seals?

I always stopped before hitting the firewall.

JHowell37

You push down all the way.

Mrmerlin

Nicole since you have a new MC installed you can push the pedal all the way to the floor as Jay says.

The reason not to go to the floor is with an older MC there is usually corrosion in the part that the seals dont wipe on and thus a few strokes and the seals will get cut and begin leaking..

So full stroke the MC and follow the procedure that was done by Jay the OP
Also make sure there is 10mm of play at the pedal this is done by adjusting the pushrod inside the car, it is rather easy to feel the MC being pushed once you work the rod

smudaaar

Nicole make sure your calipers are on the right way...your bleeders shoule be at the top of the caliper. You may have them swaped around and are unable to get the air out!

remember pump the pedal and hold pressure...crack the bleeder and the pedal will go to the floor. Do the front first as the back sets your pedal.

WallyP

smudaaar is correct...

From one of the Porsche tech manuals:

"Bleed brakes in order of stepped brake master cylinder, push rod brake circuit (front wheels) and intermediate piston circuit (rear wheels) and check for leaks. Bleeding procedures are same as for cars without ABS."

Looks like the conventional wisdom of bleeding in order of line length no longer applies.

SharkSkin

As many attempts as were made, nothing obvious was overlooked. Bleed nipples are where they belong.

I have some old bleed nipples we could drill out to try the vacuum approach.

jon928se

I guess the conventional wisdom only really applies to cars with one brake circuit. With a split system once the MC is bled there is no interaction between the circuits in theory so it makes no difference which is bled first.

In a 928 with ABS I can't find a compelling argument for there being a need to specify either left or right front (or rear) wheel first either.

Nicole

Any ideas what lifting the car high up in the front could do?

SharkSkin

Look at the MC construction. It has a floating piston(86.5+). The MC chamber closest to the booster is for the front brakes. If you bleed all of the air out of that chamber first, you will get positive displacement of the piston once the front circuit builds pressure. Trying to bleed the rears first will mean that you are compressing bubbles in that rear chamber while trying to bleed the front chamber. Less of an issue with a power bleeder to be sure, but IMHO they had good reason to recommend bleeding front brakes then rear.

When using a power bleeder, not much IMHO since a power bleeder can push bubbles a considerable distance straight down in a narrow brake line due to capillary action. Possibly this could help in the ABS block depending on how the ports/internal cavities are oriented relative to one another.

Mrmerlin

There was one hint that Nicole she said when using the press down on the pedal she didnt push it all the way down.....( The pedal)
'That tells me that another attempt should be made without using the power bleeder and after inspecting all of the hoses and the calipers for correct fitment.
BTW Dave you also mentioned that nothing has been over looked ,I would also take that as the calipers are in fact positioned correctly in that the bigger bores are towards the top of the mounting area and the smaller bores see the rotor first while driving forwards.

docmirror

Oh gosh! We talked about this a long time ago. I didn't know you weren't pushing the pedal all the way in. Yikes, that last little bit of travel can mean all the difference. It won't hurt to let the back of the car down, and have the front up for the MC and the front brake bleeding. Raise the back of the car, and lower the front when you bleed the rear.

SharkSkin

Stan, I said that nothing obvious has been overlooked. I think the caliper orientation was quadruple-checked at least twice or three times.

Doc, IIRC Nicole was pumping the pedal to the floor last time we were working on it, with power bleeder in place and while activating ABS valves/pump.