Strange issue with caliper pistons retracting during turn
 

Hi all,

it's the month for me to manage issue with my brake system


to sum up, I upgrade my 911 3.0 SC brake with the front and the rear Boxster 2.5 calipers plus the turbo Master cylinder (23mm)
no issue with the brake booster


when I drive the car in mountain I notice after hard turn an increase for the pedal travel to brake (several cm) so it's not a good feeling you can imagine.

I find a way to reproduce the issue
last brake pedal pressed give a firm feeling
I do 10 left / right fast steering move (like to warm up the tyres)
I press the brake pedal and the travel increase a lot
I press again (1 or 2 times) and it's coming back with a firm feeling

no play with the wheel bearings (changed)
caliper rebuild
new master cylinder Turbo model
you cannot imagine how many times we do the bleeding for the brake system

same situation with different brake pads


I start to consider it's an issue with the caliper repair kit and the seal (bad quality)


I would appreciate a tips / idea this issue make myself and the garage shop to become crazy

thanks a lot

Bad master cylinder ?

You mean not capable to keep the residual pressure perhaps ?

It s a new master cylinder from Porsche 2020 part

Just confirming .. with the new wheel bearings, you are sure there isn't any play? I ask because a worn strut spindle could produce symptoms similar to worn or misadjusted wheel bearings.

Speaking of wheel bearings, you checked your rears too for play, loose stub axle nut, etc?

Could it be anything to do with the routing of the flexi brake line which, if incorrect, could put some kind of undue pressure on the line and kink it or some other issue? Might be worth a look.

We triple checked for the front and rear bearing
No play

I will check for the line good idea
Perhaps the change direction can interact with …

Pull one pad, depress pedal to move the piston out a bit, pry the piston back so the pad just slips in and no more. Do the rest. Might work.

Sounds like float/runout in the rotors?
Alan

I"ve had a brake line go bad on a non-Porsche, and it gave really strange braking symptoms. I don't know of a way to check them for failure though, aside from just replacing. If they're old (or if very recently replaced, coincident with your current issue, perhaps one is defective?), maybe consider replacing.

I would replace the front brake lines flexi portion with stainless steel . . maybe if its old rubber it is expanding under pressure.

Are they floating rotors?

a common issue w/ caliper adaptors is flex. They are less than ideal in terms of rigidity.

Never knew that ...

how recently did you put these calipers in and were they rebuilt calipers.

I have had this issue quite a few times on new/ rebuilt calipers and i would consider it normal. John eluded to it somewhat

when you let off the brakes the rubber seal kit is pulling the piston back in to its resting position back into the caliper. Takes a while for the piston to realize its supposed to be out there and the rubber seal kit to lose its old memory and stop trying to pull it back . typically i've seen it take up to a week of driving, then it seems to be fine.

Mr. Bill,
Your opinion please to dig deeper.
Is it the change from radial to axial mounting of the caliper or the adapter design or ???

The fact that your adding material and fasteners in between the caliper and the hub especially aluminum is enough to add flex to the equation.

Hi
So stainless line

Solid rotor

Caliper rebuild and done 15000 miles since

I have this issue since several year

Not a problem in normal driving condition
Because I cannot feel a big change with the pedal


But with hard drive and mountain roads
It s boring and dangerous
To think

This time the pedal is firm
Or spongy …

The pedal gets spongy when the brake fluid boils and vaporizes due to dragging brakes.

No, it's just flex, the old S calipers were notorious for it.

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.