FredC

Hi all,

A friend of mine just upgraded the front brakes on his 2001 996. He went from stock to GT3 calipers and rotors. He didn't change the rear brakes nor the master cylinder. He feels that the brake pedal is very soft and that it is not a brake flush issue.

Why is the pedal soft?

Thanks in advance.

Fred

Bill Verburg

the GT3 front calipers have some what more piston volume than a 996(~11.6% more) so w/ the stock master cylinder the pedal will have more travel(i.e feel softer)

solution: install a 25.4 mm m/c from a GT3

FredC

What about the psm outlet? There isn't one on the GT3 m/c. What does he do with the psm if he does that?

jrgordonsenior

Slightly OT.... But is this for a dedicated track car? The added weight per corner is about 10 lbs. which IMHO outweighs the benefits. The OEM brakes are quite sufficient expecially if you pull some weight out of the car ...

Bill Verburg

w/ PSM doesn't the 3rd line go to the reservoir?

Land Jet

When I did this upgrade I had a soft pedal. Once the system was bled a second time it was back to normal. Unless his system is different than my turbo, he doesn't need any modification. But Bill could very well be right about the regular 996.

bsztanko

Have him cycle the abs a few times and bleed it again.

Darren

Yup...bleeding issue as the guys are pointing out. If he let the master run dry you have to bleed that too! The procedure to bleed the master is to crack open one of the bleeders, get in the car and fully depress the brake pedal 3-4 times very slowly.

Bill Verburg

turbo came stock w/ 25.4mm m/c

Darren

Totally unnecessary. Trust me, this is the setup on my car.

Bill Verburg

There's a difference between unnecessary and a better solution.

the o/p's complaint was a soft pedal because of the increase in slave piston volume. There is no question that is what will happen when calipers w/ larger pistons are used. The solution is a larger m/c. Additionally a higher harder pedal is a desirable thing(to a point), Porsche knows this and equips it's sporting cars w/ master to slave ratios that are significantly different from their general street offerings. By going to bigger front piston volume w/o changing the m/c you are going in the wrong direction.



It is quite possible that there is a bleeding issue over laying the master/slave issue as well.

Darren

Are you just repeating things or have you actually tried this exact upgrade? My car is a dedicated race car and the demands I'm putting on it are much higher than a street car.

There is NO WAY that the issue with the OP's friend is because of the ratio of the pistons, it's because there is air in the system.

Darren

I barely get any pedal travel at all by the way -- in fact I'd like more so that it would be easier to modulate the brakes. Very slight pressure on the pedal was causing ABS lockup with Pagid Blacks on the track -- I had to switch to Yellows so that it was possible to modulate!

Bill Verburg

Darren, simple hydraulics is simple hydraulics
stock 996
36/40 front calipers & 28/30 rear calipers have a slave to master ratio of 32.315

GT3
28/32/38 front & 28/30 rears have a slave to master ratio of 30.603

the Cup will have a higher harder pedal.

Incidentally, ergonomic studies have shown that a higher harder pedal is easier to modulate as long as the leg has the strength. The human leg can vary force at a fixed distance much better than a variable force over a changing distance. Thats why all RS, GT3 and most turbos have had master to slave ratios ~27-32 while the normal cars are in the range from 32 -40

Darren

Yes, I know all of this. My Acura race car has dual manual masters and it takes quite a bit of force to stop the car.

In this case it's something different. I can modulate the Acura brakes easily because even though the pedal is hard it takes about 100+ lbs of force from my leg to use full brakes. It's easy to modulate because a difference of 10 lbs of force is a very tiny change in braking force. It's difficult to over-brake at the track with that car.

My 996, in contrast, with Pagid blacks on the front (which I no longer use) has a high hard pedal but I can get FULL braking force with about 10 lbs of leg force! That's the difference. You can't modulate the brakes easily when the difference between 9 and 10 lbs is 50% and 100% braking force!