DE Brake Issue on Early Car

[Roger 911]

I have an issue when I use my car at DE events. Description; '72 911 w/ S calipers (both front and rear calipers rebuilt), SS lines, Mintex pads, and ATE Blue. For the past two years, I will bleed the brakes before a DE, firm pedel. First session is fine. When I start out for the next session the pedal is soft. After a lap or two, after I put some heat into the brakes, they firm up. Next session, same pattern, soft, warm up, then hard. This can't be normal.

I have a theory; master cylinder. I replaced the MC in '99, and the car has seen minimal use since that time. However, it sat for a period of about two years without being driven. I'm afraid that the MC bores may have rusted while it sat. Make sense? Any other suggestions?

Thanks.
Roger

[dentist90]

I have heard of a situation where after you have really heated the rotors up and stop for awhile more heat is actually transferred into the calipers while at rest. The next time you head out the fluid in the calipers is actually hotter than when you stopped, causing a bit of vapourization (boiling) of the fluid and a softer pedal. After running at speed for awhile there is some air cooling of the brake assembly (even though you are still hard on them at times) which tends to bring things back to normal. I have been told of something like this happening to a SUV pulling a camper trailer down a long grade. Brakes were fading so he pulls out at a rest stop and takes a 10 min break, jumps back in and starts going downhill again, except now he has NO brakes... pedal to the floor. Police (he lived, but you can guess what happened) figure the brakes (ie calipers) actually got hotter while he was pulled over. Go figure.
I think this phenomenon only occurs if you have a short rest between stints. I'm just guessing, but I would think less than 5mins(not enough time) or more than 1/2 hr (enough time to cool down) and this won't happen. Fact or Fiction... ?

[rdrr]

I have actually encountered the situation with no brakes after a short rest between track sessions. While driving the airflow keeps the brakes cool, but a short rest can boil the fluid. In my case it was a five minute stop after five laps on a very hot day (45C). I got in and drove slowly to the pitlane. Luckily I was the first car out because when I tried to brake the pedal went all the way to the floor with absolutely zero effect. It was very scary and I was very lucky that I had to stop on pitlane and was not waved straight out to the track! Anyway, ten minutes later the brakes were back to normal.

Having said all that, I experience the same symptoms at track days as you, i.e. nice firm pedal after bleeding, but after the first track session the pedal feels soft at the start of the second session, firming up as the brakes get hot. I will be interested to hear what might be causing it.

[dentist90]

Originally posted by rdrr

Having said all that, I experience the same symptoms at track days as you, i.e. nice firm pedal after bleeding, but after the first track session the pedal feels soft at the start of the second session, firming up as the brakes get hot. I will be interested to hear what might be causing it.

I think the key is that the pedal isn't firming up as the brakes get hot, but firming up as the airflow starts to cool down the calipers. The rotors themselves ARE getting hotter again, but there is enough heat dissipated by the moving air to more than compensate.
If this theory is true there must be many, many drivers out there who have experienced this. Perhaps it doesn't happen as readily to cars with larger, cross-drilled discs and hubungous calipers?

[rdrr]

That does sound like it makes perfect sense. However my waiting time between track sessions is about 1 hour. I would have thought that would be sufficient time to cool down.

[Roger 911]

I can certainly see how heat could be transferred after you come in, while it sits. However, there is certainly ample time for the brakes to cool between sessions (1.5-2 hrs). Is it possible that once the fluid boils in the caliper, the air is created and remains after it cools? I guess the only problem with this theory is that as the brakes heat up, they get better, usually returning to full firmness after a couple laps (Road America).

For what it's worth, I don't recall having this problem with the original iron M calipers. Is this indemic to the aluminum S calipers? I can't believe that Porsche would engineer a system that behaved liked this, especially since they kept these calipers around for 10 years.

[Roger 911]

I thought it might be nice to put some "closure" to this issue. The problem was caused by a plugged reservoir vent line. Why was it plugged? Because the genius who power bled the brakes forgot to unplug it. After the same genius unplugged the line, the brakes worked fine. I just did three track days at Mid Ohio with no brake issues at all.

And best of all...it cost $0!!!

[ianc]

Thanks for taking the time to post the solution, but I'm trying to puzzle out why a plugged vent line would cause the brakes to be soft under the circumstances you describe?

ianc

[Roger 911]

Maybe a bit more detail will help...

When I pulled the wheels and checked the brakes, the pads had about 25% left. There was also a gap between the pad and rotor of almost 5mm. This explains the long pedal travel, which essentially got longer as the pads wore down. As the pads wore down, the fluid level in the reservoir never changed. In effect, with the reservoir plugged, there was vacuum sucking the pistons back into the caliper.

As far as the firming up when hot, all I can assume is that before the pads got too far gone, there was some amount of thermal expansion, thus providing some degree of firming.

I don't know if my explanation makes any sense, but I can attest to the end result. The brakes were awesome at Mid Ohio for three straight days.

[ianc]

Makes sense to me.

ianc