996 caliper pedal feel

[Error404]

This is one for those running 996/Boxster S calipers - 996421425/6, I'm interested to hear your feedback from running your 996 calipers.

I ask because I've never been happy with the pedal feel with my 996 calipers - sure they stop the car well and hold up fine to abuse on track, but the pedal feel I have always found soft & uninspiring and requiring a lot of pedal effort to stop quickly. I swapped to vacuum servo believing the hydro system to have been the main culprit, and that did help a little but it was not a significant improvement.

I've bled the car that many times I can't count, so today I went to town with a homemade brake line pressure gauge. I isolated the master cylinder with bleed hipples in all the line ports, bled through and the pedal was hard as rock, and using all that my stumpy legs could give me I could generate about 60-70bar peak pressure without servo assistance - this stacks up pretty well against my calculation for approx maximum line pressure:

25.4mm MC = 506mm2
Approx pedal effort ~ 800N
Pedal ratio ~ 4:1
So line pressure should be (800 * 4) /506 = 6.32n/mm2 = 63.2bar

So I knew the master cylinder is fine.
Next up, master cylinder and ABS unit isolated with bleed nipples, same story pedal hard as a rock and the same pressure reading from all 3 outlet ports on the ABS unit.
I connected the rear circuit first and bleed that through, expecting the little slider calipers to be the problem. I hit the pedal again, there was a faction of pedal travel this time, to be expected - but pedal still very firm. I put this down to wear on the slider pins causing the caliper to deflect slightly. Max pressure reading at the rear axle ~ 30-35bar which seems about right and indicates the pressure regulator is working.

Then I connected up the front lines up, one at a time and followed the same procedure. I was seeing full pressure from the master cylinder of 60bar but the brake pedal was soft as I had always found before.
I wondered if the discs were warped and bowl shaped so I removed the disc and pads an put a chunk of thick wall box section in between the pistons and tried again - still soft. I put it all back together and with the help of my glamorous assistant I was able to watch as she pumped the pedal. It looked like it was a combination of the pad compressing slightly and the caliper body itself flexing

So am I expecting too much? What are other peoples feedback on this set-up?

[twoqu]

Sounds like the Monobloc vs 2 piece bolted construction argument!
One thing is evident. Porsche did start to use calipers with integral pad bridges on later models.

[AndyP]

Mine are the same, like you say you have to give them a good shove to get them working and even then they don't seem to have any bite or feel. I've never driven a 996 but I find it hard to believe that they would feel like this in a factory Porsche.

[twoqu]

http://www.stoptech.com/technical-support/technical-white-papers/monobloc-vs-2pc-calipers

I have also heard of a modification replacing the OE pad pin with a HT bolt!

[newsh]

Could still be air.. I've had 4 bikes with Brembos and the best way to bleed is from the bottom up, either by pumping fluid into the bleed nipple with a syringe or pushing the pistons in as far as they will go then pumping them out again. In fact I did this yesterday on one of the bikes as the lever felt soft. Not sure if there's enough "movable" fluid in the caliper to clear the line back to the master cyl reservoir on a car though.

TBH I think my 996s are probably similar to yours.. Are you on DS2500s? I wonder if these are the problem, mind you I had Textars when I first fitted them and they were awful.

[AndyP]

I'm on DS2500's at the moment, but I have some Pagid S sport pads to try next.

http://www.design911.co.uk/fu/prod12...-99735193904-/

[bertman]

Think I bought up some of the exact same issues a while back Alex.Think it was a combination of running a racing type pad and just being different to most cars I have driven.
Basically I found it was a case of get used to the feel because it is never really that constant,not like it is in a vacuum servo car.
I even had the caliper flex etc,check my old post mate

[Henkjan]

Mine are the same, like you say you have to give them a good shove to get them working and even then they don't seem to have any bite or feel. ....

same here, no bite, but the feel is OK (to my taste that is), able to dose the braking properly

[Error404]

I'm running DS2500s. Can you elaborate on your bleeding technique John? Like Andy I fail to believe Porsche would have accepted this on the 996

[Monkeymagic]

im running Bosch Pads on mine and found them to be much better than the ferodo's I had originally. I dont get a lot of pedal travel but it is a very positve feel and the bite is very good when required, nice and progressive not snatchy like modern brakes.

[Rusty]

Sounds about right. None of the controls have much feel, so I just press until it stops and at least it dies compared to stock

Are your 996s upside down like mine are?

[Error404]

They are the right way up, small piston leading

[s2driveruk]

i've recently changed the pads (DS2500) and discs on mine and given them a thorough clean and they are working well. I only have issues with mine when they haven't been 'serviced' in several months, i.e. pads removed, pistons eased fully back, pad guid pins cleaned up and pads and pins covered with ceratec paste at contact points.

You may have glazed the pads? pull them out, get a clean wire brush and pass it over the pad surface a few times then refit and see how they feel. Also, have you got them up to temp, the ds2500's do need some heat to work effectively (hence not a fast road pad due to lower them oem initial bite from cold) I was staggered how they come into their own on a track event once there is real heat in them, plenty of feel and modulation...

I guess the flex issue is usually banded about by Stoptek (typo?) who used to always have tons of stuff about 2 piece versus single block. I have seen mine flex when someone is helping bleed brakes and it does look like there is a fair bit of movement, however these are so well proven across a multitude of road cars that I find it hard to beleive there is an issue after this length of time.

Porsche probably weighs less, has a different level of assistance versus as 20+yr old car and their disc thickness (i think) was 28mm not 30 so tha 2mm *may* be important - not as though they couldn't fit a 30mm but chose not to.

Just my 2p

[s2driveruk]

what lines do you have on the front btw...s/s flexis or a combo of rs2 hardlines and an oem rubber line from strut to body?

[Cottis]

Never ran 996 calipers on the S2 but with both the Movit setup and then the AP's they always needed some standard driving (more so on the AP's) about to get up to temp before they properly stopped without really hanging on the anchors.

Do you still get the same spongy feel once they are well up to temp? If so (and I agree with you that the orig 996 probably braked well) then might it have something to do with not running the OEM discs. Or you might be for all I know but everyone seems to run the A8 disc and then the 996 calipers and then a variety of pads. If you put the exact same 996 setup full on your car, it would probably improve. As your tests show, the braking setup on the car is sound. Has to be one of the components and it has to be discs or pads IMO