cornerhans

I have just updated my rear brake calipers on my 1990 C2 using the 92 and later 4 piston calipers in lieu of the 2 piston calipers that came stock from the factory.

Everything bolted up with no issues but I am now ready to install the new brake pressure regulator that is nessary to provide the correct bias between front and rear.

The size of this thing is about the size of a shotgun shell and I am wondering if anyone has done this upgrade and could tell me where it is located in the system. I did not see it under the hood anywhere. I am assuming it is under the car somewhere between the master cylinder and the rear brakes.

Any help would be appreciated.

Rocket Rob

I recently installed 4 piston calipers on my 1991 C2. I changed the brake bias valve too. Its an easy change. Its located in the trunk next to the ABS pump, in the trunk. You need to remove your spare tire and the black plastic cover of the ABS pump. The bias valve is oriented vertically and is on the left-hand side of the pump (when looking down into the trunk). Remember to re-bleed the brakes when you change it.

garrett376

Here are some pics to help you find it:
https://rennlist.com/forums/964-forum/204795-ever-wonder-how-the-proportioning-valve-works-and-what-its-innerds-look-like.html

cornerhans

Rob-

Thank you very much. I just wrapped things up and it went well.

cornerhans

Rob and Garrett-

I really appreciated your help today, enough for me to finally join Rennlist.

cornerhans

apyip

Hi Rob, Can you share which brake bias valve you have changed to? What bar rating? Do you have the part number? I had changed my calipers few months back but have yet to replace the bias valve. The braking still feels terrible.

Rocket Rob

I installed a bar bias valve rated at 55 bar. Its the standard valve on the C4's and late C2's. The part number is 964-355-305-10

LouZ

apyip,

Can you elaborate on your feelings? Even when I tracked my 90 with 2 pistonrears, I felt that the brakes were excellent! Now with 4 piston rears and the 55 Bar bias valve, it responds to the slightest touch.

Did you ever change the rubber brake lines @ the 4 axles? After 18 years, they could be soft and bulge when you apply the brakes.

PorscheZen

apyip,
Did same change for myself back in Nov. 07. Never changed my bias valve and breaks run great at the track. It could be the pads you are running...

I've noticed that when I hit the brakes when I'm parking (slow movement) I hear a "clink" sound coming from my rear brakes. Mechanic said it might be the aftermarket pads - but I changed since then . Now I'm thinking it's the pistons need to be re-built. Any thoughts?

apyip

I have since replaced all brake disc with cross drilled Zimmerman. I have also changed all brake pads and installed S Steel braid lines.

Whenever I brake, my whole car dives forward. This is why I am now keen to change the bias valve, hoping this will sort out this problem.

By the way, what pads are you using?

apyip

I think you might be right about the pads. I am also using aftermarket pads and they seem crab to me. Moreover, with them squeeling all the time when I brake, it kind of gets you on the nerve.

Also, I notice that that the brakes don't bite as much when cold. especially on reverse. I do not know if that is the pads' fault or could I not have enough vaccum in my system?

boxsey911

On changing to 4 pots I developed a similar noise. Like yours, mine does it only at very low speed but I would describe it more as a 'pop'. I get the noise regardless of what pads I put in (I change them for trackdays). At first I thought it was air in the lines but bleeding hasn't cured it. Currently I also think it is the pistons making the noise, as they resist the initial low braking pressure and then 'pop' as they move. In my case they are the older 928S4 calipers which don't have dust seals which might be of some relevance?

jimq

Mine make the popping sound after I changed them as well mostly on the first time I use them after sitting for a day or two.

Steven C.

I did this change with 993 rears and no popping sound. Did you guys put the factory nushrooms back in? I put new mushrooms in and cleaned everything well and then greased all the mating surfaces. I also used Hawk HPS pads as this is a street car.