mattatk

Dear All,
Had two instances of my Boxster S (986) brakes locking up on their own whilst driving.


First time, coming out of a tight corner accelerating, PSM OFF & ABS lights came on as the car decided to brake to a halt without me?!
Brakes locked on solid. Thought it was faulty PSM & disconnected battery & waited 5 mins.
Once battery was reconnected brakes appeared to work normally...

Second time, driving with son casually & car again decided to brake to a stop by itself? No PSM or ABS lights. After waiting for about 15minutes the brakes appeared to loosen off. Drove home cautiously, but the brakes appeared to be dragging a bit.

At home tried the Bentley manual booster check & it appears to be working fine (pump & hold pedal down & then start car, pedal drifts down a bit further as vacuum builds). Tried the master cylinder check & the pedal seems to hold its position (maybe drifts down a bit?).
No sign of any leaks, except from last week there was a little bit of moisture around the fluid reservoir, which I've sealed up with silicon whilst troubleshooting.
Bled the brakes as per Wayne & Bentley manual. Clean brake fluid came out without bubbles.

Have also been having trouble with clutch hydraulics (suspiciously related??) but I am pretty sure that problem is a master cylinder also.

As I live in Australia I am trying to minimise the car's down time, do I replace the master cylinder &/or booster ?

Macster

Any chance you went for the clutch and hit the brake pedal instead? A bit of track red mist? Well, I had to ask.

Hard to imagine a scenario where the brakes apply themselves. The only way I can think of this possibly happening is through some fault in the ABS system.

When you bleed the brakes unless you have the diagnostics computer to cycle the ABS system you do not flush/bleed the fluid from the ABS system.

This system is quite complex and designed to work with clean brake fluid. Porsche recommends a brake fluid flush/bleed every two years. This is a full brake system fluid bleed (and which includes the clutch should the car be equipped with a manual transmssion -- the clutch hydraulic system shares the brake fluid) which if one does the brake fluid bleed himself or even takes the car to a shop that is not aware of what a full brake system fluid flush/bleed entails, the car's brake ABS system over time accumulates brake fluid contaminated with water.

I'm not sure how this can affect the ABS system but it can't be any good for it.

Thus, I believe you ought to consider starting out the car with a full brake system fluid bleed including the ABS system and the clutch hydraulic system.

I'd run plenty of fluid through to make sure all the brake lines/reserviors/etc. are fully flushed of any old fluid and this is replaced with clean/fresh fluid. The proper type of course.

Also, I'd be tempted -- cause of course the importance of correctly functioning brakes -- to after the fluid change have the brake system and especially the ABS system checked out with whatever diagnostics the Porsche diagnostics computer provides for this system.

Depending upon what this check out turns up or fails to turn up sort of determines what you do next.

If nothing untoward turns up perhaps the fluid was the problem? In this case then -- but it is up to you it is your neck etc. -- you can drive the car and see if the brake system behaves itself. If it does, then adhere to the that every 2 year *full* brake system flush/bleed schedule.

If the brakes act up again then there's a hardware problem that hopefully can be identifed by the tech from the symptoms and further diagnostics.

Sincerely,

Macster.

mattatk

Thanks for the reply.

Definitely not me pressing the wrong pedal. Second event, I took my foot off all pedals & looked in astonishment as the car pulled up heavily?!

Car us nine years old & great history of Porsche servicing & Nine Plus servicing (independent Porsche mechanics, very good rep). Owners service manual shows the standard 2 year fluid changes.

I changed the clutch unit with a Sachs clutch six months ago myself due to slippage. Wouldn't bleed with a Motive pressure bleeder, so I suspected a blockage & pumped clutch pedal in the process (I know, should keep pedal depressed). Subsequently bled whenever clutch pedal was up, instead of down ?! That's why I suspect a faulty clutch master cylinder.
I was wondering if the two problems were related, as they both use same fluid system?

Macster

Well, as I mentioned if the brake fluid flushes/bleeds didn't include exercising the ABS or PSM system these portions of the hydraulic system contain old fluid.

I skimmed over the ABS/PSM section of my Turbo manual and it states after the brakes are flushed/bled "as usual" then "bleed at the front on right with running booster pump".

However, this is for my Turbo.

My Boxster reference says that after installing a new brake master cylinder or a new hydraulic unit do the following while bleeding: Open the *right* rear bleeder valves, then fully depress the brake pedal several times. Hold for 2 to 3 seconds after each depress, the slow release the pedal. Repeat this procedure in teh sequence of rear left, front right, front left. This removes all bubbles from the primary circuit.

Also, do the above if the brake hydraulic system largely empty or if after bleeding residual air can be detected by excessive pedal travel.

There is of course a caution for older cars with higher miles: Double the number of pumping cycles and use only *half* the brake master cylinder stroke. Failure to do so can damage the brake master cylinder/piston.

In addition vehicles with TC (traction control) consisting of ASR and ABD (4-channel system), the ABD circuit in the hydraulic control unit must be bled when the hydraulic control unit is replaced/removed..

As for the clutch hydraulic system my info is these must be filled/bled using a pressure filling and bleed device. (Motive Power Bleeder is what some use and is what I have.)

The bleeder valve must be opened sufficiently and the pressure around 1.5 bar.

*Before* the system is filled or bled the clutch pedal must be in the fully depressed position.

The clutch pedal must be raised manually from this depressed position.

If the system was opened for repair/replacement the minimum filling time or bleeding time must be 30 seconds.

If you did not do the above, my advice would be to redo the brake system bleed and clutch system bleed and do it right this time, according to the book.

Then see if the brake behavior changes for the better.

Since the brake system and clutch hydraulic system share a common reservior the symptom may be related to the what I believe is an improper clutch system fill/bleed but I do not know the dynamics well enough to explain it or offer good evidence it is not can not be related.

Lastly, the ABS unit receives a signal from the brake light switch and the speed sensor signals (one at each wheel). These are used to know when to activiate the ABS system.

These signals are continuously monitored and if a fault detected the control unit switches the ABS function *off* and turns on the ABS warning light and stores a fault code in non-volatile memory of the control unit.

Additionally at the start of every trip and upon a speed of 6kph being obtained a test program is started. The soleniod valves and pump motor are electrically activated and checked. If a fault is detected the ABS function is switched off and the ABS warning light is turn on and a fault code is stored.

Absent an ABS warning light -- assuming the bulb is not burned out or missing -- I would hazard a guess the problem does not lie with the ABS system but lies in the more basic/fundamental brake hydraulic system.

My earlier point about the lack of full brake fluid changes due to failure to exercise the ABS during a bleed may be wrong. However, I believe my Porsche tech buddies have told me every brake job done at the dealer gets this treatment. My printed references are I know not complete and I do not have the many addendums/corrections that have been put out since this hardcopy printed. The techs get these all the time and have electronic copies too boot.

Sincerely,

Macster.

mattatk

Wow, thanks again Macster for your input, I really appreciate it.

Unfortunately I'm off on holidays (fortunately?) for a couple of weeks so I can't take this any further yet. I'll post the findings/solutions as I get them on my return.

I had read about the brake light affecting the ABS/PSM, but they work fine & also read somewhere that the MAF can affect the PSM (?). The second brake event didn't include any warning lights.

As for the clutch bleed, I did it as advertised, except for the pumping of the clutch pedal under pressure (20psi). That got the fluid going but I think I may have damaged the clutch master cylinder internally (no external leaks). Which allowed it to bleed with the pedal up. As I understand the system, pushing the pedal down ports fluid to the slave, releasing the pedal allows the fluid back up the line. So no fluid should flow out the slave bleed with the pedal left up (except in my case!). I reckon I've busted an internal o-ring or some sort of sleeve.

I think you may be spot on with the first clutch bleed causing the problems with the brakes. The overpressuring of the reservoir, possibly by the Motive bleeders 20psi & pumping the pedal may have caused the slight leak in the reservoir or attachment gaskets. This would introduce water into the system through absorption. Over 6 months of use, the water may have worked its way into the ABS/PSM & caused these problems. Otherwise the water expanded in the normal hydraulic lines & applied brake pressure on all wheels (as all wheels were dragging, not sure which one(s) locked up completely though).

Thanks again Macster.

fpb111

"No sign of any leaks, except from last week there was a little bit of moisture around the fluid reservoir, which I've sealed up with silicon whilst troubleshooting."???


Sounds like you are pressurizing the reservoir when the fluid warms up. Unseal the reservoir so that the fluid can flow freely back into the reservoir when you let off the brakes /use the clutch.

mattatk

fpb111, I haven't sealed the reservoir from the system.

The gaskets that attach the reservoir to the master cylinder were weeping slightly (slight moisture present) so as an interim measure I put silicon around the outside of the gasket-reservoir joint.
The fluid can still flow freely from the system into the reservoir.

Or are you saying that the system is meant to be able to leak, in order to remove excess pressure? I'm guessing that I've misunderstood you here, as the fluid is very hygroscopic.

fpb111

No leaks needed or wanted just vent to atmosphere. It sounded like you may have sealed the cap/reservoir vent. If the reservoir were sealed it could build enough pressure, not much needed, to restrict flow from calipers/clutch slave back to the reservoir causing brake drag & clutch anomalies. And possible leaks around the gaskets attaching the MC tto the resevoir.

Had this happen at Pocono one year. One of the guys helped me flush brake fluid with a pressure bleeder and in the rush to get back on track we forgot to take the plug out of the vent line.
The brakes would start to drag and lock after a few hot laps. It took us a lot of head scratching to figure why.
As the brakes/fluid got hot there it created enough pressure in the topped up reservoir to keep the pads from retracting which created a vicious feedback loop, hotter brakes more expansion needed, none allowed by plugged vent= hotter brakes etc...

mattatk

fpb111,
That sounds like my problem! Not sure where the vent line is though, I'll look into the schematics. I've misunderstood the system. I've ordered a new reservoir, cap & attachment gaskets anyway (as the gaskets were leaking).
Unfortunately takes a couple of weeks to get the parts to my place in Australia (& my holidays), so I'll post the results at the end of the month.

Thank you VERY much for your input, it has been a relief to discuss the issue & hopefully solve it. I owe you one fpb111!


As for the clutch, I've pulled its master cylinder & looked inside, the seal looks like my dogs been chewing on it. Not sure if that was due to age (9yrs old) or me overpressurising the system with a pressure bleeder & pumping the pedal.
As it is stuffed, I've dismantled it & will post some internal pics for interest sake. It appears to work fairly similar to the master in the following link:

http://www.4g63turbo.com/tech/eclips...djustment.html

Macster

I like the explanation offered by "fbp111", better than I like mine based on my suspicion of contaminated/old fluid in the ABS hydraulic system. That you report the gaskets were leaking suggests an overpressure situation exists.

Best of luck and be sure to post what you find. It is a rare symptom, brakes coming on with no application of the brake pedal.

Sincerely,

Macster.

mattatk

Update:
Got back from holidays & fitted new reservoir, cap & grommets to master cylinder.
Pulled the vacuum line to booster & checked inside with torch & small mirror, looked very healthy.
Flushed brake fluid with a different super DOT 4 (incase previous brand was faulty). Bled brakes - a few bubbles came out - but otherwise uneventful.
Brakes appear to work beautifully? Tested PSM on subsequent drives & it all works well. Still haven't bled the PSM but as I don't have a PST2 or PIWIS, I'm going to hold off unless problem returns.
Does anyone know if Durametric software can activate PSM on 985 S?
I'll post if any more develops.
Thanks for the input.