Brakes fade completely after 16 minutes of driving

[John91186]

So i found this epic twisty road and drove it two times in a row and when i ended my second "lap" and started driving "normally" for some 2 minutes in the end of the road, the brake pedal wenīt all the way to the floor.Had to use the handbrake to stop to a bus-station.The pedal going completely to the floor is weird but even weirder is that the pedal was as stiff in the end of the 2nd lap than in the start of the 1st.How did the brake fluid possibly over-heat so quickly?And is my brake booster working as intended?It was scary to think after wards would wouldīve happened if the pedal would have hit the floor in one of the tight bends....

I was thinking of going for a track in the end of this month but after this i am not too sure.

here is a video of the 2nd lap:https://www.youtube.com/watch?v=EcoNUbNN7x0&list=UUVRXwEQeeE6xgPOhmVKTrCQ

It does not look too fast but you can hear the slight understeer i got at 2:59
so i was pushing it as fast as i could with no track experience and only 4 months of driving experience.+ i was in the brake 70% of the time because all of the blind curves.

[KTL]

Are you sure it was the fluid overheating? If its the original brake master cylinder, it could simply be failing you. A friend's 1986 Carrera recently had the master cylinder no longer work. His car is street driven and rarely are the brakes used hard. But nonetheless the master cylinder no longer would hold pressure and the pedal would go to the floor.

How old is the brake fluid? Are the brake hoses original? What type of brake pads? Do you possibly have a dragging brake caliper? All of this must be checked. Brakes are the most important system on the car and you need to be sure they are in good operable condition.

The brake booster does not affect the pedal in a manner that would make the pedal fall to the floor. The booster provides effort assist to reduce the amount of effort required to apply the brakes.

Being on the brakes very frequently (70% is quite a lot) and having aged fluid can indeed boil the brake fluid. Brake fluid absorbs water and therefore very old fluid can boil quite easily. Water boils at 100C and that is a rather low temperature in terms of brake fluid within a brake system. With enough water absorption, it is easy to boil the fluid. It does not take a lot of boiling/air creation for the pedal to drop.

Typical fresh brake fluid boils at around 250C and you have a much higher resistance to boiling with fresh fluid.

[GH85Carrera]

How old is the brake fluid?

I change my fluid every sing spring. No exceptions.

If you don't know how old it is, it is too old.

[Dueller]

Bet you have a dragging caliper that's overheating the fluid

[dentist90]

Dragging caliper is easy to diagnose. Just drive nicely around the neighborhood for 5 mins trying not to use the brakes much. Pull over and touch each rotor with a finger. The one that burns the tip of your finger is the bad one.
Failure as you describe would be two things: stuck caliper and contaminated fluid. I had a dragging brake that I could smell when I parked in the garage but no boiling of fluid as it was flushed just a year ago.
Here's a hint: if tomorrow morning your brakes feel normal again (after cooling down) it probably is an overheat situation. If you pump your brake pedal several times until firm and hold it down hard it should not slowly sink to the floor. If so there is a leak somewhere, either in the lines or in the master cylinder. Check your fluid level as well.
Good luck.

[kodioneill]

Old brake hoses will swell up inside and choke off the return of the fluid. (very common).

[John91186]

Originally Posted by GH85Carrera

How old is the brake fluid?

I change my fluid every sing spring. No exceptions.

If you don't know how old it is, it is too old.

The car was in a major maintenance this spring at my local porsche service center and they checked the brake parts and everything was workings as intended and the front pads are porsche oem pads.Brake fluid was changed in the service,not sure to which quality tho.Rear brakes were dragging for 2 weeks after the service,noticed it after some 500 miles of driving.Got the problem fixed and some random pads installed to both rear brakes.

Could some braided brake hoses fix the problem?The discs are fine,so are the pads and the other brake parts were checked in the service so i am pretty sure it is the fluid overheating,it is just so weird that when i was driving fast and braking every 2 seconds the brake pedal kept its stiffness and feel but when driving normal speeds and not braking for 2 minutes the pedal went to the floor.Later i discovered that pumping the pedal 3 times to the floor made the pedal work normally but after some 5-10 seconds of not braking,the pedal wenīt to the floor again...

And after cooling the brakes for 5 minutes i continued driving and the pedal felt almost normal,after 20 minutes of highway driving it felt completely normal.

[dentist90]

Originally Posted by John91186

Brake fluid was changed in the service,not sure to which quality tho.Rear brakes were dragging for 2 weeks after the service,noticed it after some 500 miles of driving.Got the problem fixed and some random pads installed to both rear brakes.

Time to dig a bit further. Doesn't matter what type of pads you're using, if they are dragging you will have premature brake fade as there is no cool down period on the affected caliper. It only takes 1 caliper to have vapour in it and you will lose pedal firmness.
You say you got the problem fixed... was it caliper rebuild, new calipers and/or new brake hoses? Just changing pads won't do it. When mine was dragging (front right... I could tell by the heat in the rotor) there was plenty of pad material remaining, but the inner pad was ~30% more worn than the outer pad. The inner piston seal was stuck and the pad wouldn't retract, hence the dragging brake. Needed new calipers or a rebuild if you're handy. It could just as easily have been a swollen/blocked brake hose that prevented pad retraction. Don't know until you get in there. Stainless braided brake hoses look nice, but OEM rubber ones work just fine. More than enough to lock 'em up.
It is possible that under the heavy braking you were doing the calipers wouldn't release fully, whether it be bad seals or hoses, and the fluid didn't 'boil' until some time after you had eased up. Normally the brakes would cool when you travel without using them, but if even ONE of your brakes doesn't release properly it will not cool down and eventually, well... you know.
Try driving for 10 mins without braking much and check the temp of your rotors. They should feel cool, maybe a biot warm. If any of them feel hotter than the others there is something going on. If the calipers are old or haven't been used for several years it may be more than one.

[gliding_serpent]

Just a thought. If you get everything hot, and then suddenly stop moving as fast (especially if you really slow down or stop) you can get heat sync into your brake fluid, boiling it and causing fade. Happened to me once at the track after going into the pits without a sufficient cool down lap. Took too much time checking my tires/wheels and holy-crap minimal brakes after i went into turn one.

[michael lang]

After watching that video I'd say that road was a deer impact waiting to happen. Nice road though. Can't imagine having to be in the brakes very much on that road. It didn't look that bad except when the road dropped off down the long hill. As far as your problem is concerned, I'm with the test of the guys, sounds to me like you have a piston hanging up causing a loss of fluid which weakens the pedal pressure. As far as why the pedal pressure stiffens again the only thing I can think of is the temp drops creating a vacuum that either pulls the caliper piston in or releasing it. You didn't say, unless I missed it as to whether you pulled the wheels and inspected inside, and what you found. It is possible that since your mechanic did something to the rear you now have an issue with the front and not because he caused it just because who knows how long it has been since the front seals were last replaced. That's where I'd start if I were dealing with that problem.

[John91186]

Originally Posted by michael lang

After watching that video I'd say that road was a deer impact waiting to happen.

And because of that and the bikers,i was dragging the brakes all the way around the corners.And i will test if one of the calipers is dragging as soon as i have a change,i have an ir temp meter which is just the right tool for that.I am not sure what they did to the rear calipers at the service but i am pretty sure they didnīt do any bigger maintenance to the calipers asides cleaning them and pushing the piston inside...

And i didnīt inspect anything because my mechanical knowledge of my 911 is limited to changing chip to the ecu.