dsddcd

I am hoping someone might have some experience with this issue. I have a 14' Cayman that has a Brembo GT front and rear brake kit. Since the install there is "Slop" in the brake pedal in which the first inch or so of the travel is light and produces no braking. After this the pedal is good and everything works well. We changed the master cylinder to a GT4 cylinder in hopes it was a displacement issue and this did not help. In addition the ABS was bled, using a PIWIS, and the brakes have been bled a number of times since. I also checked the calipers and there is no knockback of the pistons.

The pedal has no play in it and I am guessing the booster is good as it worked great before the updates. So I am lost as to what the problem might be.

BondJ

What you describe sounds normal to me. Swapping out discs and pads will often change the initial bite-point on the pedal. Any spongy feeling should reduce as the pads bed in.
As the pedal and bite are fine beyond the initial ‘slop’ and given you’ve checked the obvious problem areas - calliper piston movement, air in the system etc - I’d just continue the bedding-in process.

TMc993

With due respect to BondJ's statement about bedding the pads, an inch of pedal travel with no bite is a lot of pedal travel with no braking and to me at least, much different than sponginess...

Even so, what you described sounds like the booster is having to build pressure in order to activate the master cylinder. Everything downstream from the booster is new and you state that there's no play upstream in the pedal linkage...That leaves the booster.

dsddcd

Thank you for the Reply's!

The brakes have several track days so the bedding is not the issue. The only thing I can think of is that the booster has a gap between the master and the push rod that might be causing it. I was hoping that someone else may have had the issue with the GT3 Master and stock booster. I know in the older car the GT3 master was a bolt on but don't know if that is the case for the 981/991's.

BondJ

No experience with that combo but, given you’ve already bedded in the brakes, certainly worth looking at. Booster issues usually show check-light warnings with abs throwing a wobbly. And the brake pedal start point can be higher.

dsddcd

I read many hundreds of posts and I can find anyone that has had the same issue, it appears that as long as you get the correct style of master, integrated brake switch or not, the rest is a complete bolt on.

The mystery continues!

BondJ

As I understand your original post, the problem occurred AFTER you changed the discs and pads but BEFORE you swapped out the master cylinder? This is why I’m doubting the booster. If you hadn’t touched it at this point why would it fail? Your later post in which you mention the gap between the master and pushrod would indicate that the problem occurred AFTER you swapped out the master cylinder. In which case you may have disturbed something.
Also, did the brake kit come with bigger callipers? If so a larger bore master with a matched booster maybe the answer. My previous experience with uprating brakes is with older non-Porsche cars so I’m guessing here. But I don’t see why the principal would change. Maybe talk to a good indi or even Brembo?

steved0x

Do you have titanium brake shields (since you mention track). If they get any bend in them, it will cause the behavior you mention since it pushed the pistons back.

tjsmaryland

When you bled the system with the PIWIS, did you have the system pressurized to 30 psi minimum while having the brake pedal cycled? I have bled the brake system on my GT4 on 2 occasions with a PIWIS, and I have found maintaining the system pressure at at least 30 psi during the process to be essential to getting all of the air out of the ABS circuits.

dsddcd

Sorry for the confusion the car came to me after both the BBK and the master were completed, the customer was not sure when the issue started with the pedal as he did not notice it right away but believes it was after the master was completed.

Nada, just track pads with no shims.

Very interesting, so I have only pressurized to 1.5 bar then used a vacuum device at the wheel. Typically I will only open the bleeder enough to barley pull any fluid without the pedal pulsing but still get good flow with the pedal. Going to 2bar always scared me as I have had caps blow off at 1 bar and it makes a HUGE mess, hours of cleaning! I will crank up the regulator to 2 bar and give it another go.


Thank you all for the advise!

tjsmaryland

The reservoir cap will likely blow off if you pressurize to 2 bar, unless you secure the cap to the master cylinder with some sort of clamp devise (ask me how I know, I blew one of my caps off in the dead of winter and ended up hosing my car down in 35 deg weather to get the brake fluid off the paint work). I machined a fixture out of aluminum that allows me to secure my reservoir caps to prevent another blowoff.

The instructions in the PIWIS maintenance screen state to have someone pushing the brake pedal up and down while the system is pressurized and the bleeder valve on a brake cylinder is open. The sequence is left rear, LF, RF, RR. I used 2000 ml of brake fluid when I bled my ABS in January. It is a pain in the backside process, but it works.

ncster

I might be mistaken but I believe the service manual recommends bleeding with a pressure bleeder on the reservoir rather than a vacuum bleeder. I've only ever just bled the brake lines but not the ABS unit so not sure if it makes a difference but might be worth a shot.