jstyer

So... Trouble in paradise.

My 993's brake pedal has been sinking. The first couple presses of the pedal give little resistance and then the pedal starts to sink. If I press the pedal multiple times in quick succession, I can get the pedal to firm up and the brakes become effective. So I was thinking classic master cylinder problem... However before I diagnosed anything, and seeing the car was sitting for almost five years, I flushed over a gallon of fluid through the brake system to get out all the nasties that had built up. I used a motive power bleeder, and bled RR, LR, RF, LF as recommended, and I bled both valves at each location.

Unfortunately the pedal did not firm at all, and in spite of getting a lot of junk and waxy substance out of the brake lines, there was no change.

Next step was new master cylinder. I removed the old one, and when I opened it up the cylinder walls and seal were both in amazing condition... a bad omen. In spite of the master cylinder that looked perfect internally, I went ahead and replaced it since I'd already bought one. One more thing to check off the list. I re-bled all of the brakes. And found a dirt road to lock the wheels up and cycle the ABS pump. In spite of activating ABS several times, and driving for about half an hour, the pedal felt exactly the same.

Which brings me to the ABS pump. Does anyone have any fixes for a faulty ABS pump? Is paying to have it rebuilt or purchasing a new one the only option, or would it be possible to rebuild it myself? The reason I ask is because I found a post by Arena993 (mike) from back in 2009 where he mentioned a five dollar fix. The thread can be found here:

https://rennlist.com/forums/993-foru...-abs-pump.html

I tried to PM and email Mike two weeks ago to no avail... I tried again tonight and we'll see if he gets back to me.

Which brings me to my next potential solution. Would it be possible to just put a T off of the front brake line from the master cylinder and an extension from the master to the rear brake line and bypass the ABS pump all together? I won't be driving this car in adverse conditions, so can I ditch the ABS?

Any info would be great!!!

pp000830

I add my own, Have you considered the Brake Booster?
993-355-023-10? Could be the booster or just one of the lines going to it. Since you have ruled out the master cylinder and it seems to me a failed ABS wild not cause peddle sink as it is part of a closed system. Im not an expert here but you may have a much simpler problem than you are inclined to investigate.
Andy

k722070

which car/brake system?
vacuum or hydraulic?

993/907

Had similar problem about 5 years ago; 8 out of 10 times brakes were fine, then pedal would go half way to floor.
Took indie about six weeks to chase it down.
He explained it as some kind of bypass valve located in the rear of the car.....didn't fully understand when he explained it way back then, but has been fine ever since.
Someone on the board with a grip of how the brake system works might be able to fill in the blanks.
Gordo

bobt993

As mentioned you can have a couple of sources of problems. If you have a vacuum leak this could cause poor pedal feel (usually a harder pedal though). At the motor on the left side (driver) is the vacuum line that feeds the booster in the front of the car. Easy to quickly check that this holds vacuum after building some. There is a check valve on the vacuum tree that also can be checked. The ABS system can be checked using a code reader for faults. You really need a Bosch hammer to purge the ABS pump if there is any source of air in the pump itself or crap. To do a complete flush this needs to be done since using the power bleeder does not cycle the ABS pump. I can post a diagram of the ABS system, but I would first to some diagnostics on the OBD port. You also going to want to clear the fault codes on the ABS system and check for codes a second time after running the car. As far as bypassing the ABS pump. Not that simple. You would need to setup the brake system for fully manual. The ABS helps a bunch with pedal feel and is part of the way the brakes on the 993 operate.

Texas993

You didn't mention it in this post, but didn't this car sit for several years before you got it? That may make a difference.

You mentioned waxy substance. Many 993s ago, I had a Targa with similar issues. In the end, that waxy stuff was the problem. If the brakes are not flushed regularly, moisture builds up in there and the wax forms. My Targa had a ton of it. My mechanic did all sorts of things and was about the replace some expensive ABS parts. Then he discovered that he could flush the system with the former DOT fluid and it would break down the waxy substance.

Routing was to flush with DOT 2 for a few days. Then replace DOT 3.

Worked for me.

jstyer

Thanks for the responses guys! To answer the questions so far, the car is a 1995 C2 and so it uses the mechanical system, not the vacuum system. And the sinking pedal that has to be pumped up occurs on every single stop. Not intermittently.

As mentioned previously I first flushed the car with an entire gallon of DOT 3 fluid and let it sit for two weeks with the new fluid. I then flushed it again and the brakes felt the same. Replaced the master cylinder and re-bled... Still no change.

Arena993 gave me a call this morning and was a big help... Using his suggestion I removed the ABS pump and fashioned up a wiring harness and switch assembly that allowed me to supply 12V power to the ABS pump system and cycle all of the valves individually while flushing it with a solvent and compressed air. I didn't notice any contaminates come out... and when I reinstalled the ABS unit, and again re-bled, the sinking pedal was totally unaffected. I literally felt no change.

I will definitely try the DOT 2 fluid later if it does in fact have better solvent properties. In the meantime I'm going to install a series of brake fittings between the master cylinder and main brake lines so that I can bypass the ABS and see if I notice any change. If I do, then it would seem like the problem is definitely in the ABS. My questions about this are as follows:

1. Can I just Tee off from the single "front" line leaving the master cylinder into the two front lines that occur after the ABS unit?
2. Will the difference in size of the front two brake lines that occur after the ABS unit effect the brake differential between the two front wheels? (e.g. front left line is bigger than front right. Will this matter?)
3. What does the reducer/valve looking fitting that goes between the ABS pump and the rear brake lines do?
4. Is the proportioning handled by the master cylinder, or internally in the ABS pump?

I know this is a lot of questions guys... But I just want to try everything before I go and drop a couple grand

On the upside, I may be slowly becoming the worlds best brake bleeder... Or maybe the world's worst. Since I've yet to actually fix anything

Justin

Luftd993

The C2 has a vacuum system (brake booster, brake servo or whatever you prefer to call it) I’ve had these cause the pedal to drop on other cars.

The ABS has one valve that regulates the pressure to the rear wheels unless you have the ABD option in which case there are two valves.

Have you got the PET catalogue on your hard drive? Here is a link if not.

http://www.porsche.com/usa/accessori...artscatalogue/

It might help you to understand what you have got.
Good luck.

bobt993

As David mentioned unless you have a C4 or RS, the brake system has the brake booster which is located next to the steering rack at the front of the car. I think it is likely you have 3 channel ABS on a 95. You can simply count the fittings leaving the ABS pump in the front trunk. You can also look at your option codes on the car or the actual ABS module which is near the nose in the front. Bosch will list the fault codes on the system if there are any and rule out the need for some of the troubleshooting methods your intending to try.

ninjabones

Sounds like it could be a brake booster issue to me... Try a little test. With the ignition off, pump the brake pedal several times until the pedal feels very firm... then while still pressing on the pedal, turn on the ignition. If the booster is functioning properly, you should feel the brake pedal sink as the booster actuates.

jstyer

Haha, thanks for the hug, and the feedback!

1st of all... you guys are exactly right. It is a vacuum system. I guess my misunderstanding of the mechanism of what a booster actually does made me think it was mechanical. The car is also a non ABD/LSD option car. So as mentioned, only one rear brake line. That being said, I realize that their are internal valves on the ABS pump that regulate the side to side braking on the front lines, but what actually handles the proportioning between the front and rear wheels? Is that handled directly in the master cylinder, so that the two lines coming out of the MC are already proportioned pressure wise? Because it seems like the two lines are completely separate within the ABS pump system. So if it's not the MC, I would guess that there's something in the ABS that acts as a throttling valve... even during normal driving conditions. (ABS not activated)

I think the ABS pump assembly would be easier for me to understand if I had a schematic of just the ABS pump... Do you guys know if BOSCH has a schematic or any kind of drawing that shows what that pump looks like on the inside?

I will definitely try the brake booster troubleshooting technique you suggested Glen...

Jeff96-993

I assume you bench-bled the master cylinder prior to installation. I missed it if you mentioned it earlier.

jstyer

Actually... I didn't bench bleed prior to install.

Due to the fact that the reservoir is not on the MC, and I wasn't sure how I'd manually push in the MC piston with it under the car... I just didn't do it.

My thinking was, that if I was using a pressure bleeder, the air would have to be pushed out of the MC by the new fluid before travelling to the rest of the system, correct?

jhg41977

If you ever have an accident and its found that you bypassed the abs I would think you would be in a very bad position for defense.

jstyer

That may be true... But even if I bypassed it for 5 minutes and just drove down my driveway and back, I'd be able to tell if the pedal firmed up. If so I'd know it is the ABS pump and I'd be able to send it off. Rather than sending it off for $700 to get rebuilt with nothing but a hope that that is where the problem lies.

I'd definitely prefer to have the ABS. Not necessarily for lawsuit protection, but for my OWN protection! The way I see it, $1000 is a cheap price for additional safety. But, I want to be sure I've identified the problem before wasting the money.