It's true that in extreme repeated use w/o let up the fluid can boil and cause a long or even no pedal but in street use that's not likely. The diagnostics related above seem to show that heat isn't the issue.

pad knockback from flex also causes a long pedal. A few jabs reextends the pistons to where they belong as a result a normal pedal.
full Boxster break set w/ 23/8mm m/c should have a nice high and hard pedal at all times. Unless there is air in the system, or gas from boiled fluid or pad knockback of the pistons.

Not an issue with fading or boiling liquid

Because it can occur with cold brake

Very good article here

https://nasaspeed.news/tech/brakes/pad-knock-back-what-it-is-how-it-happens-and-what-to-do-about-it/?amp

They discuss about residual pressure valve to add on the circuit

I don’t know where to by it

Generally the only time a rpv is used w/ disk brakes is if the m/c is mounted lower than the slaves

in a disk system the inner seals provide the retraction force

It really sounds like some play in the system - either flexing or something loose, allowing the rotors to float under high load. Doesn't happen with quiet driving so not run out.
It would take very little knock out to cause a long pedal. If each pad knocked out say 1mm, that would be a lot of fluid to push 4 pistons (front calipers) back on to the pads = long pedal.
Presumably as soon as that first pedal push happens, the 2 nd would be rock solid. The slack has been taken up - until next time.
So I am guessing the problem won't go away until the source of the movement is cured.
Alan

Well regarding the price I will install 2lbs residual pressure valve
At least it s just a benefit

And if the problem is still present
Rebuild the caliper

I hate it when "It s boring and dangerous".

before doing anything I'd want to find out why thus is such a problem w/ yours when most others don't have that big an issue.

I have never been a fan of the Boxsters on a 911 as it solves nothing, but it also doesn't generally cause issues either.

Do some cornering or whatever causes the problem, let the car roll to a halt without braking, or safely stop on the handbrake, then:

Carefully without burning yourself, try to insert feeler gauges between the rotors and pads.

Measure the diameters of your flexible hoses.

Measure the temps of your braking system at various places.

Examine calipers for movement or wobble.

Then pump the brake and confirm it had long travel during the measurements.

Now repeat all measurements with short pedal travel.



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You should not need to add a residual pressure valve to a 911. In fact I tried adding one once after reading an article like the one above when building the brake system for my racecar. It caused my brakes to lock up once the system got warm, because it wouldn't allow the brake fluid to backflow to the master cylinder as it warmed up and expanded.

My SWB is sitting in a shop for the non S Boxster upgrade. Stopped by there last week and they mentioned the calipers had to be tilted to get the bleed valve to the very top. I can plumb so made sense. I also made a mistake in my younger years so something to think about. My guess is the Boxster mounting tabs are different than what we have.

Re the braided lines. I went back to rubber when I went through the stock setup. Couldn't even get the tires to lock up. Back to braided now. Hopefully I get the thing back Mon or Tue as we have a dry spell. I like my floor pans so H2O other than a bath is a no.