Strange issue with caliper pistons retracting during turn

[florent78]

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

[yelcab1]

Bad master cylinder ?

[florent78]

You mean not capable to keep the residual pressure perhaps ?

It s a new master cylinder from Porsche 2020 part

[stownsen914]

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.

[stownsen914]

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

[Ian Comerford]

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.

[florent78]

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 …

[john walker's workshop]

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.

[Alan L]

Sounds like float/runout in the rotors?
Alan

[stownsen914]

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.

[rowingone]

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

[911obgyn]

Originally Posted by Alan L

Sounds like float/runout in the rotors?
Alan

Are they floating rotors?

[Bill Verburg]

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

[yelcab1]

Originally Posted by Bill Verburg

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

Never knew that ...

[theiceman]

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.

[3rd_gear_Ted]

Originally Posted by Bill Verburg

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

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 ???

[Dpmulvan]

Originally Posted by 3rd_gear_Ted

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.

[florent78]

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 …

[john walker's workshop]

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

[Bill Verburg]

Originally Posted by 3rd_gear_Ted

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 ???

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