Eric Kessel

Just a quick "is this normal" question. I changed out the rear calipers to 92 964 rear calipers, and replaced the proportioning valve on my 90 C2. When driving around, the pedal feels fine, but when I first get in the car in the morning, the pedal is really hard. Thanks for info one way or the other. (Just a little paranoid about the brakes.....)
-Eric

joey bagadonuts

Eric,

Could you describe the feeling in a bit more detail. When you say hard, does that mean it engages without much downward travel? And once engaged, is there limited downward travel?

My brakes engage gradually and travel about 2/3 the way down to the floor before locking up. Unless I'm running them hot at the track, the feel is fairly consistent.

I have the stock C4 setup which should be similar to your system. Is the proportioning valve the same as the C4's? Did you install new rotors and pads? When's the last time you flushed the fluid? Do you have stainless brake lines?

A little more information might help identify the variables and lead to a reasonable explanation.

Eric Kessel

Before starting the car, I put my foot on the brake pedal, so when I start it without the parking brake it won't roll. The pedal will not move at all until I really "stand" on it....... then it moves to where it feels like it's engaging the pads...... strange feeling...... I put the proportioning valve that is called out for 92 - C2's, I don't know if it's the same for a C4.

I had to drain the system when changing the valve, and rear calipers, and pressure bled it. I put in new rear pads, but front pads, and all rotors looked very good.

The car has the original rubber hoses (with 90k miles), and I did change the two rear hard lines that attached to the calipers, since the connectors were rusted to the lines when I went to remove them.

I put super blue fluid in, and have driven about 750 miles since the brake work. I'm thinking I'll bleed them again...... when driving around, the pedal feels okay...... not as smooth as my old 944, kinda notchy at the beginning of travel.

Thanks for any info.
-Eric

Adrian

Dear Joey,
The C4 brake system and the C2 brake system are quite different. The C4 uses hydraulic boost and the C2 uses vacuum boost for a start. Master cylinder of the C4 is larger and the list goes on.

Dear Eric,
The reason your pedal feels hard I believe is a direct result of your modifications. This is caused by the fact you have increased the amount of pressure being allowed to the rear brakes as compared to before; 33 bar originally and now 55 bar but you are using all the same system components. The amount of fluid being delivered to the rear brakes is now more for longer. This causes a pressure drop in the system and the pedal will start to go hard.
These modifications have changed your braking bias and the feel has changed because the load on the system has changed.
You just have to get used to the feeling that is all.
I am not surprised either that without vacuum boost that you have a hard pedal. Please remember this when using your brakes without the engine running.

Please everyone remember that brake modifications require some thought. Often such modifications are done with worn front brake components. I recommend that these modifications include replacement of the front brake discs and pads as well.

Ciao,
Adrian
964C4

Eric Kessel

Adrian,
The front rotors and pads had been changed within 10k miles (before I bought the car, but receipts, and looking at them they look great). And, I put on the correct rear calipers to match the proportioning valve.

Was there something else I should change? (I'm planning on replacing all the rubber lines, just due to age.... but the rest of the components seem okay.... )
Thanks,
-Eric

Adrian

Dear Eric,
As I explained to you in our email exchange I have some concerns which were general in nature and not aimed specifically at your modification. Your problem however is a good test case.
As for rotors looking good that is not a good indication. You have to measure their width. Mine looked good and were below limits. They were replaced this week.
I also said I have not researched any of the subtle differences between the 91 and 92 C2s in their basic brake system. The master cylinder is the same but I wonder if they changed anything else. I know the reservoir was changed at some time.
I am concerned with the installation of the 55 bar rear bias valve. This allows a maximum of 55 bar pressure to be transferred to the rear brakes up from 33 bar. Your ability to generate brake pressure has not changed so more is heading rearwards than before. These things get a little tricky but need to be looked at.
We do need some feedback from yourself to tell us if you feel a change of brake bias towards the rear.
You have reported a problerm and I would like to be able to work out what the actual cause is.
Ciao,
Adrian
964C4

Eric Kessel

Adrian,
Since I did the brake caliper/valve upgrade, I have driven an autox, and a track day, along with a few weeks of commuting. The front wheels are filthy with brake dust, and the rear wheels have a very light dusting, not really even noticable.

I haven't noticed much of a change in the brake bias of the car. Also, the car is at stock US (SUV) height, with 90k miles on the suspension, so there is a bit of nose dive during "aggresive" braking. (I had driven two autox's before the brake upgrade, and it doesn't seem to have made much of a difference.)

I put metal master pads on the back, and the front already had some type of track pad (from the braking at autox, and sqealing in general). So, the braking seems more than adequate.

Again, i'm only asking to see if an extra firm pedal before starting the car means anything significant (like start saving $$$$$$$$$$ for a master cylinder, which is whittled by hand by elves.........)

Thanks,
-Eric
90 C2

Christer

I would like to say that I don't think you have anything to worry about. Most cars do the same thing, plus the fact that a master cylinder is like UK£70 or so. Why worry about what your brake pedal feels like when you are stationary with the engine off???? For the record, I have carried out exactly the same modification as you, but did NOT change the PV valve. My braking is fine except after repeated (i.e. more than 30 mins) braking from 140mph down to 50 in order to navigate the various roundabouts. Other than that, I can confirm that the brakes retain their excellent feel and power, although an upgrade will be necessary after the aforementioned extreme braking scenario caused brakae fade and I almost filled my pants (as well almost died)....

Adrian

Dear Christer,
I am beginning to wonder if replacing the bias valve is such a good idea. I need to do some more research and need engineering data which might be hard to get. The amount of brake pressure available is dependent on the rear brake circuit piston in the master cylinder. I am beginning to wonder based ont he comments of Eric and yourself that the maximum pressure available to the rear circuit may only be circa 33 bar maximum. Therefore fitting a 55 bar bias valve may well be a waste of money.
It is the inside of the master cylinders that I will have to try and get into to check my theory and that might need some smooth talking.
Ciao,
Adrian
964C4

DaveK

I'd certainly back up Christers comments. I've driven his car (since he changed the calipers) and I was actually very impressed with his brakes indeed.

Adrian

Dear Dave,
I have no doubt you are happy with the brakes on Christers car. However that is not the point. You are comparing cars based onpersonal feel. We have Eric who has a concern and I am afraid I do not subscribe to "you have nothing to worry about" until we can genuinely determine he has nothing to worry about. What Christer has done to his car which is somewhat modified cannot be compared via discussions via email. It is hard enough trying to find a cause via the internet.
Brakes is a safety item and we have not lost anyone yet and I have no intention of starting now. If Eric is unhappy he will remain unhappy until he can be convinced or find a reason for his unhappiness. I am unhappy because the pedal firmness technically should not have changed so there has to be a reason for it especially seeing as the front brakes were not touched.¨
I just had my brake rotors and pads changed (drilled versions) and it is like I am driving a new car in the brakes department. This does not help Eric though if you understand what I mean.
Ciao,
Adrian
964C4

Adrian

Dear Eric,
I should have asked you this as my first question. Silly guy I am sometimes.
What part number rear callipers did you use? The correct ones are a 928 part number. Which rear brake rotors (disk) did you use? The 951 or 965 part numbers.
By the way according to Porsche you have to replace the rear brake bias valve with 55 bar unit from the C4 which is a 930 part number. The 33 bar can only be used with the 2-piston version. I am working on exactly why.
Ciao,
Adrian
964C4

Eric Kessel

Adrian,
I purchased the rear calipers used from a PCA member that was parting out a 92 C2. I didn't actually look at the part #'s before putting them on the car.
I ordered the 964.355.305.10 pressure regulator that is spec'ed for C2/RS America's from 92 forward.
The rear rotors were in good shape, so I didn't change them. I did buy a set of Metal Master pads, and they fit in perfectly.
I still have the pressure regulator I removed, and will look at the number on it later.

My concern for this post was whether a hard pedal on startup meant a leak in the system some where (vacuum.... I've checked out any places for fluid to leak, every thing looks fine), or a failing master cylinder....etc.....
Thank you for all your effort on this.
-Eric

Adrian

Dear Eric,
I really need to know the part numbers of the callipers you installed. Same for the brake rotor. There are certain matching part numbers of callipers and rotors.
I have discovered one of those unexplained things. For the ROW Carrera 2 from 1992 and up they use part number 930 355 305 01 brake bias regulator. This is the same as the C4.
Only the Carrera RS uses part number 964 355 305 10.
However in the USA the RSA and Carrera 2 use 964 355 305 10.
Both are 55 bar units but I wonder what the difference is?
Ciao,
Adrian
964C4

Eric Kessel

Adrian,
Later this week, when I switch the wheels, i'll find out the actual part numbers off the calipers. (I know the calipers have 28mm, and 30mm pistons in them, since I ordered and installed replacement seals)

The rotors were installed before I bought the car, but were working fine with the little 2 piston calipers. Is there any measurements I can take from the rotor without removing it from the car?

-Eric