Brake Pedal Goes Soft After 7ish Minutes On Track

[StantonGTI]

Hi All,

 

I've done some searching and have not found concrete answers. Some history on the vehicle/issue.

 

It is a new to me 1999 C2 with 115K. No PSM. Just before my purchase a new brake booster was installed because the car had a soft pedal at the track. The issue was diagnosed by a Porsche dealer, but was installed by the owner previous to me. The owner had some issues getting the system bled, so he took it to an Indy to get it done.

 

Once I took delivery of the car, I drove it 1-2 days per week on the street with no real issue, although now that I think about it, I may have had one hard braking situation where the pedal got soft.

 

Fast forward to this weekend which was my first track weekend since purchasing the car. After about 7 minutes on the track, the pedal got real soft, almost going down to the floor. It was still braking, but I didn't have a ton of confidence in it, so I came in. This is on Porsche Pads with Stock Rotors.

 

I can't believe I got the fluid too hot since it was the first time I was every driving a 911 on the track (previous cars were a miata and two e36 M3s), all with very different driving dynamics, so I was taking it easy, in a lower run group, and I was at a small track with only 5 turns with just one hard braking zone. It was also on 40 degrees. 

 

Since I was not sure what type of fluid was used after the brake booster swap, I did a full fluid flush with SRF using a motiv bleeder (starting from right rear wheel, moving closer to the MC).

 

Took it back onto the track, same result.

 

Came back, and rebled, using the two person method (I had read that the power bleeders could cause an issue).

 

Took it back on track, and same result.

 

I read that the ABS pump may need to be bled, so I plan on buying a durametric, so I can do that (although I also read it does not always work, and the dealer is the only place that can bleed).

 

One thing of note that I do not think would matter, but two of the bleed nipples were stripped. I was able to use vice grips to do the bleed, just thought I would mention. I do plan on replacing all of the line (although they are only 3 years old), and all of the nipples.

 

Any thoughts on what I should do next? Don't really want to throw parts at it. Also, any thoughts on how I could test without actually being at the track/getting arrested for doing a bunch of threshold braking on the street?

 

TL:DR

- New to me car, stock pads and rotors

- Previous Owner replaced brake booster due to soft pedal, had Indy shop do the bleed

- My first track weekend, pedal would get soft after about 7 minutes (not very hard on the brakes)

- Flushed and bled with SRF and MOTIV Bleeder

- Happened again after next session

- Bled again using manual two person system

- Still happening

 

Thanks in advance for the help.

 

 

[wizard]

I would suggest that you have a leak somewhere. Have you checked the fluid level after pumping the brakes?

[StantonGTI]

I would suggest that you have a leak somewhere. Have you checked the fluid level after pumping the brakes?

Yes. Fluid level is stable. Actually when I first checked it was a bit over the Max line. When I flushed, I did make sure not to exceed the Max line.

[JFP in PA]

First of all, pressure bleeding does not cause problems, we use it all the time in the shop without issue.

 

As you are uncertain of the braking system, I would replace the bad bleeders and lines, then try bleeding it with the ABS/PSM system activated, just to be sure there is no air in the system control network.

[StantonGTI]

First of all, pressure bleeding does not cause problems, we use it all the time in the shop without issue.

 

As you are uncertain of the braking system, I would replace the bad bleeders and lines, then try bleeding it with the ABS/PSM system activated, just to be sure there is no air in the system control network.

 

Thanks JFP in PA. Car does not have PSM (at least has no button for it), but I will try that. As far as activating the ABS, does that need to be done with a durametric, or is there a procedure for doing that with it?

[Loren]

You are correct no PSM on a MY1999 C2.

Yes, you will need Durametric, a PST2, or PIWIS tester to turn on the ABS pump during bleeding.

 

Also, besides looking for small leaks you can put a small pressure on the system and then use a gauge to see if it leaks down.

[StantonGTI]

You are correct no PSM on a MY1999 C2.

Yes, you will need Durametric, a PST2, or PIWIS tester to turn on the ABS pump during bleeding.

 

Also, besides looking for small leaks you can put a small pressure on the system and then use a gauge to see if it leaks down.

 

For checking for leaks, would putting the Motiv Pressure Bleeder on it, and taking it to 20PSI and leaving it on over night work to check for leaks?

[JFP in PA]

 

You are correct no PSM on a MY1999 C2.

Yes, you will need Durametric, a PST2, or PIWIS tester to turn on the ABS pump during bleeding.

 

Also, besides looking for small leaks you can put a small pressure on the system and then use a gauge to see if it leaks down.

 

For checking for leaks, would putting the Motiv Pressure Bleeder on it, and taking it to 20PSI and leaving it on over night work to check for leaks?

 

 

It might, but it may also mislead you somewhat as the braking system operates at hundreds of PSIG, which the Motive system cannot duplicate.

[Paul Waterloo]

I would also consider your brake pads. If you are not running track pads, it's easy to get them hot in a hurry and then you'll have no brakes and it will have the symptoms you suggest.

[StantonGTI]

I did some more digging. I found a couple of things. Not sure if any of them could be the cause.

 

1. The electrical connection to the brake fluid reservior is cracked and and loose ( I can see the connection inside of it). I assume this is just a level sensor, so I am ruling that out for now.

2. The ABS pump is missing some of it's mounting points and is kind of just floating there. The bracket is only held in by two of the four bolts, and the pump is not secured to the bracket. This is obviously something that needs to be fixed, but not sure it would cause the issues. Anyone know the correct bolts to secure the ABS pump to the bracket?

3. There was a bit of fluid on top of the lines going into the ABS pump. This seems to go against gravity, so I am guessing some may have dripped down there as I was bleeding the system this weekend.

4. I did put the Motiv Bleeder on it for two days. The pressure dropped by 2lbs. Not sure if this is an accurate reading, or if it could be due to temp swings we had over the last two days.

5. I drove the car for the first time since Sunday, and the pedal is rock hard. When I was at the track and I let it sit between sessions, I thought I was getting the full pedal back, but after driving it today, I realize I was not.

 

My next steps:

1. Replace all bleed nipples

2. Replace all soft brakes lines

3. Secure ABS pump

4. Bleed system using durametric (will be here on Monday).

5. Swap to some more aggressive pads (although I really don't think it was that as I was not on the brakes that hard).

 

I am also considering swapping the ABS pump. I've seen a few other cases where people had the same issues over the course of season, and it ended up being the ABS pump although nothing really pointed to that. They are currently selling for $50-$100 on eBay, and seems easy to swap. I figure if I am playing with brake fluid next weekend, it's better to do it all at one time. 

 

Any other thoughts?

Edited May 19, 2016 by StantonGTI

[DBJoe996]

For 2. - #6 and #8 perhaps on the bolts...not sure what you are missing

http://www.autoatlanta.com/porsche-parts/hardparts.php?dir=996-99-05&section=605-00

[StantonGTI]

For 2. - #6 and #8 perhaps on the bolts...not sure what you are missing

http://www.autoatlanta.com/porsche-parts/hardparts.php?dir=996-99-05&section=605-0

Awesome. Thanks!

[StantonGTI]

Hi All,

 

Thanks again for the help. Now for the next question. What is the proper procedure when using a durametric when bleeding a 996 with ABS but no PSM? I can find the tutorial for PSM, but not non PSM. Thanks!

[JFP in PA]

Hi All,

 

Thanks again for the help. Now for the next question. What is the proper procedure when using a durametric when bleeding a 996 with ABS but no PSM? I can find the tutorial for PSM, but not non PSM. Thanks!

 

The procedure's are exactly the same for both systems.