do a gravity bleed on the ABS if you dont have a power bleeder,
then go back start at the MC and do the foot pedel bleed.
if these parts have air in them most of it should come out via gravity

 

Thanks so much Stan! It will probably be a few days until I am able to get to this due to me being busy with work, but I'll report back once I give your instructions a try.

 

Simon--

I went through a bunch of your dilemma after I accidentally pushed air into the ABS unit. Tried the conventional bleeding methods including the power bleeder, gravity bleeding, and the two-person pedal bleeding methods. It got a little better but still half-way down and not as firm as before. I could "pump up" the pedal too, a sign that there's still air in the system.

Ultimate solution was a careful vacuum bleed, from low to high end on each section of piping in the system. I evacuated the ABS unit from the MC side, and pushed fluid uphill from each corner with the power bleeder. It was a bit of a chore but the pedal is once again up where it's supposed to be and firm again. It took the better part of an afternoon with the car up off the ground to get to the various fittings. Particularly important seemed to be the rear splitter T, which sits in a local high spot.

Clues to the process needed came from an experience Nicole shared several years previous to my episode. After several visits to local shops, she followed a recommendation to someone who was able to get things right again using the combination of methods.

 

Thanks dr bob, I read your suggestions in other threads related to soft brake pedals and think that this might end up being the ultimate solution to my problem as well. I don't have a tool to vacuum bleed the lines though, so I am going to try Stan's instructions first and then go from there. Just out of curiosity, what tool do you use to vacuum the lines?

One thing that sounds different from your experience compared to mine is that I can't pump my pedal up. It always engages close to the floor. I can't pump it up higher but I can't press it all the way down to the floor either. Without the car turned on it's rock hard. Don't know if that points to anything different, but just thought I'd throw it out there.

 

I used a vacuum pump to evacuate the sections. Vacuum hose pieces slide over the connections and tubing ends, and hemostats to pinch the little hose sections at each wheel. I also did a "bench bleed" on the MC with little recirc pipes going back into the reservoir. The later MC has a bleed/vent port high in the front chamber for getting air out, but none for the rear chamber past the port from the fluid reservoir. It was a bit of work, but not too bad as I integrated it with a system refurb effort that included new hoses, caliper rebuilds, plus pads and rotors.

----

Your description of a slightly low pedal at all times is interesting. Verify that the pushrood "freeplay" is correct at the pedal as a start. Follow up with a two-person effort where your trusty and not visually-impaired assistant can help you see where the pads start moving vs pedal position.

Also -- Worn wheel bearings, crud in the hub faces, etc can cause rotor runout that pushes the pads back when the system is relaxed. Do a quick check of at least front wheel bearing play at 12 - 6 o'clock position with the wheel up a little to rule out that worn-bearing possibility.

 

I tried bleeding the MC and ABS lines as Stan suggested but I never was able to get any air out of them, just drips. Bleeding the system at the wheels produced no bubbles either. Just for kicks I put the wheels back on and drove the car around the neighborhood--up and down the steepest hill I could find and also getting up to speed on flat roads and doing some hard braking. After doing that I bled the system again and it produced a fair amount of bubbles, but the pedal is still soft. If the system is as it should be I should never have any bubbles, is that correct? If so then it looks like there is probably air trapped in multiple places in my brake system and the bubbles work their way out when I drive. Either that or there is air that gets into the system somehow, but I don't see any fluid leaks.

Well, it looks like I either keep driving it around the neighborhood (I've already done this twice with the same results), or just move on to dr bob's method and vacuum/pressure bleed each line from low to high. I *think* that I understand how to do it, but I might be wrong. Do you just disconnect each line one at a time from low to high and vacuum the fluid out, put everything back together and then push fluid through each caliper? If so, on which wheel did you start and what order did you use?

And I'm not sure what happens when it comes to filling the ABS lines. If there's nothing in there but air and I'm pushing fluid through there from either the calipers or anywhere else, won't that just be pushing air towards the ABS, or is the hope that it would "burp" itself up towards the MC bleeder port?

Sorry for my ignorance but I just want to make sure that I don't have to do this a bunch more times. I've already bled this car enough for a few lifetimes.

P.S. I checked my front wheel bearing play and there is very slight movement on the LF wheel at the 12/6 position, but that was the case when I bought the car a year ago and the brakes were fine then. After paying close attention I can kind of pump up the brakes, but barely at all. All signs seem to point to air in the system, but what do I know.

 

what fluid are you using?

 

I already had a bottle of unopened DOT 4 brake fluid on my shelf so I just used that (Castrol I think, but I'd have to check). I know that you mentioned to use ATE but I my local stores didn't have any. Is it possible that a substandard brand of fluid would somehow cause air to get in the system?

 

Simon,

Having replaced the master cylinder, having bled it forever and got nowhere and if the brakes work but you are feeling a soft pedal and pumping does not obviously firm up the action then my suspicion would be that you have a brake hose that has gone soft and what you are feeling is the brake line [or lines] swelling under pressure thus needing more pedal. Stock brake hoses do this to some extent but ones on their last legs more so. Whether you can see anything obvious by visual inspection remains to be seen. If there is a general age related degradation it may not be quite so obvious.

I did mine last year with my wife pushing the pedal and had no issues and considering what you have done it is difficult to see what else could be causing you problem.

The above may explain your symptoms but not how you noticed the onset of the problem. If you do not know when your brake hoses were last changed then maybe it is time to pre-empt any possible problem in that department.

 

Yeah I am beginning to feel like I might as well replace everything that is replaceable on the brake system, especially stuff that is bound to fail at some point. Soft brake lines are probably right up there on the list of things that I might as well change out while I'm going through this ordeal. Everything was working fine before I bled the system, and since then everything's gone awry. To be fair I did most of the bleeding before I changed the MC so maybe the car just needs more bleeding now that the new MC is in there, but I have bled it at least 5 times since it was installed and it hasn't made my pedal any firmer. Bubbles do come out after driving it though, so I don't really know what to think anymore.

 

Your pedal should feel firm when the engine is off [no servo] and if you press the pedal and start the motor you should feel the pedal go soft and depress a little as the vacuum builds.

If you had air in the system I might expect the pedal to feel a little squishy when the engine is not running.

As per my previous post I have no explanation as to why initially you had no problem and after changing fluid this occurred assuming the master cylinder replacement played no role in the case history.

Have you had any other 928 owner test the system - always good to eliminate the possibility of "OCD"!

 

Even though the pedal is currently "hard" with the engine off, there were a couple of instances where after bleeding I felt the pedal with the engine off and said "this definite feels harder and higher than before", but then I turned the car on and the pedal sunk to the same low level as before. One of those times was when I replaced the master cylinder. After driving around a bit and turning the car off the pedal returned to the same hardness as before--it was hard, but not quite as high as I had just felt.

So related to what you said I guess that even though my brake pedal is hard when the engine is off it could be even harder. I don't know if that really helps at all, but just trying to fully disclose any and all data points.

No other 928 owner has driven it in this state, but I'm confident that this isn't imagined (no offense taken whatsoever by your question though). My dad has driven it before and felt the pedal now and commented on how low it is.

 

OK lets try this,
please follow these directions,
swap in a new MC,
replace the flex lines with SS versions,
Use ATE fluid,

If you you do all of these instructions ,
then you increase the chances your brakes will be perfect for a long time

 

Simon --

Received your PM.

The Castrol fluid (or any other DOT4 fluid) will be fine. The ATE is a known good fluid, and matches the brand of the MC so there's no chance of incompatibility. Meanwhile, I rotated among two ATE colored fluids while they were available in the US, and the Castrol GT LMA fluid that was readily available. Where I live now there's no local Castrol, so I have been using a Valvoline Synthetic DOT4 for the last several years now. Stay with that Castrol and you'll be fine.

About bleeding and bubbles: The little bleeder screws have a coned end that blocks the end of the bleeder port in each caliper chamber. The threads don't do any sealing normally. When you are pressure or pedal bleeding, it's possible for air to enter around the threads and make a tiny stream of bubbles in your bleeder hose.

There's a cardinal rule in bleeding brakes that the receiving container is always higher than the caliper, and the end of the hose in that container is always submerged in fluid. I use an old brake fluid bottle, hole drilled in the cap for the clear hose I use. I added a wire bale to the bottle so I can hang it up on a spring coil. There's then a continuously up-sloping path for bubbles to follow, a liquid 'trap' to keep air from being somehow drawn in, and usually enough back-pressure from the liquid-filled hose to minimize the chance of somehow drawing air back into the caliper.

If I see more bubbles than I expect, I sometimes add a smear of silicone grease around the bleeder screw threads on the outside. Not at the tip where there's a risk of contamination of the fluid, just enough to block casual air flow around the threads. Vaseline will do in a pinch.

In times past, ATE used to include instructions with their replacement MC's that included "bench bleeding". They offered a little kit of fittings and hoses that would connect to the ports on the MC while it was clamped in a vise. Hoses route back into the reservoir, ends submerged in the fluid in the reservoir. (catching a theme here?...) Then the piston would be manually stroked a few times with a screwdriver to purge all the air out before the new unit was installed. Then you'd install it with those little fittings and hoses still connected, taking them out one at a time and immediately connecting the line from the car. In our cars, they added a bleed port for the front chamber right on the MC. Unfortunately, the discharge ports for each chamber are low on the sides so the rear chamber (drives front brakes) can hold some air at the top front if it isn't cleared by the bench bleeding.

There are a couple high spots in the hydraulics that offer places for air to hide out. The ABS unit is one, the T between the two rear pipes is another. I suspected that my ABS unit was harboring fugitive air after a mistake while swapping fluid. As mentioned previously, I used a vacuum pump (one I use for AC system service when needed) with a liquid trap to draw the air out, then allowed fluid to be drawn back in by the vacuum. I metered the fluid back in from the low end of each line, using some vacuum tubing and a hemostat at each pipe end. The T at the back was opened so I could make sure no air was captive where the pipe from the front of the car and each branch to a rear wheel connect.

A pocket can form at the top of any rubber brake line too. You can solve the last one by pressure bleeding with the caliper unbolted, hanging so the line slopes up to the caliper, comes in low on the caliper with bleeders at the high points. The crossover pipe between the caliper chambers is low so won't pass air if there's much fluid at all.

This is tedious, but a methodical approach to purging each section will get you there.


FWIW, the systems are evacuated of air and filled into vacuum at the factory.

 

Thanks so much for the suggestions everyone, I genuinely appreciate the help. I'm going to scrounge up the necessary tools and parts and try to get rid of this problem once and for all.