Adrian

Dear Eric,
According to Porsche the rotors used for the 2-piston callipers are not to be used with the 4 piston callipers. You can measure the overall thickness of the rotor and let me have this measurement.
Ciao,
Adrian
964C4

Bill Wagner

Eric:

The C2 uses a vacuum assisted brake booster. When you start up the car after it's been sitting for a while, it's typical for the vacuum booster to leak a little, and essentially lose all of it's "boost" capabilities until the car is started again. This will leave the pedal VERY hard to push. On most cars, this is completely normal. On a typical car with standard vacuum assisted power brakes, it's generally normal for the pedal to be hard after the car has been sitting overnight, and then once the car has started, "soften up" as the boosting capabilities of the system kick in.

Since you've made some mods to your car, here are some things to ask yourself:

1. Are you ABSOLUTELY SURE this condition didn't exist before hand and you just didn't pay any attention to it or don't remember it?
2. When did the problem start? Right after you changed the proportioning valve, or after you put in new pads?
3. Have you turned the car off and then pumped the pedal a few times to see how well it's holding vacuum? A typical vacuum assisted boost car pedal effort will start to become "hard" as you pump the pedal with the engine off several times, and then when you turn the car on, become almost instantly "soft".

My own personal guess (AND THIS IS A GUESS) is two fold. When you put the new pads in, you decreased the amount of distance the pedal could move when unassisted because the new pads are thicker, and thus gave you the impression that the pedal effort is higher, when in fact it's just shorter. Additionally, the new proportioning valve is now redirecting differently than it was before, thus once again, you get the feeling that this is different.

Personally, if I were you, I would meet with a Porsche specialist, tell them exactly what you did to the car, and let them test drive it. It probably wouldn't cost much, and I know that the guys I deal with would do a quick check like that for free. As Adrian said, brakes are a safety issue and nothing to really fool around with if you're uncertain. We can't feel you're pedal effort before or after the mods you did, and with that in mind, let someone who CAN evaluate the car properly check the system out for you.

Hope this helps,

Adrian

Dear Bill,
I agree with you entirely and we had sort of said the same thing. One clear issue that is emerging from this discussion is that modifications are being done piece meal. By this I mean there are a set of components which work together as a team, callipers, rotors, bias valves, pads. If one or more parts of the team are left out who knows what the end result will be!
In this case the rotors should have been changed. I do not know the difference between a 2-piston rotor and a 4-piston rotor but obviously there are differences otherwise Porsche would not have used different part numbers.
Ciao,
Adrian
964C4

Christer

Adrian

Just to add, I did not change my rear rotors either when the upgrade took place. At the time, nobody told me any different so I had to take the decision based on that.

Just to reiterate, whilst my car is slightly modified - the brakes are as follows:

fronts: completely standard for 1990 year.

rears: used 1991 up rear calipers, with the 1991 up pads, but with the existing rotors/discs as fitted to my car before the upgrade.

I did not change PV valve.

Eric Kessel

Adrian,
I just looked through the PET program, and the rear rotors listed to go with the early 2 piston calipers is the same 951.352.041.02 part number given for the later C2/RS America rotor. Do you have the part number for the two different rotor?
-Eric

Bill,
I changed the pads at the same time I changed the calipers and valve. The pedal feel definitely changed (only when the car is off) when I made all these changes. I had the car inspected for the track right after these changes, and my mechanic said everything feels fine..... Of course, the car had been driven for the 45 minutes to get there...... I'll make an appointment to leave the car overnight, and see what they think.

Adrian

Dear Eric,
I will have to get back to you. I have a few questions I am awaiting answers on. There seems to be differences in ROW and USA and differences in some of the various Porsche manuals I have. This is getting confusing but it is worth to nut it out. I have sort of put things together and you are correct. The rear brake rotor is the same for the C2 and C4. The differences occur with the turbolooks (big difference in price as well). From what I can see Eric you have carried out all the required changes to make the modification work properly. The harder pedal may well be the result of this. What I recommend before you spend any money is that you check out somebody with a 92 or 93 C2. Not a tiptronic because some of these even in 92 were still fitted with 2-piston rears.
Ciao,
Adrian
964C4