I have a question about my 1990 964 c4, I wonder if members could be so kind as to give me advice.

The previous owner upgraded the calipers on my recently purchased 964 c4 to the larger 928 S4 “big blacks”. It looks like they have the original c4 brake master cylinder, it’s labelled as an ATE product 964.355.025.52. The brakes have worked really well until a Porsche Club Autocross I participated in over the weekend... the brakes lost their hold…

If I apply heavy pressure to the pedals they will hold firm but if I apply light to medium pressure they will sink slowly to the floor. Someone has suggested that this is an internal leak in the master cylinder.

My question is what part (number) should I replace this broken m.c. with? It’s funny, according to the PET document the factory part for the 964 c4/Turbo is listed as a different part number than the one in my car. It’s listed in PET as part 964.355.903.00??

So, would you suggest I stick with the original factory part number 964.355.903.00 or upgrade to a 993 m.c.(do other parts need to be upgraded if I do the 993 m.c. upgrade at the same time?)?

Any advice or tips from you would be greatly appreciated.



The existing (now failed) master cylinder in my car

 

I would get the upgraded 993 master cylinder. Also, I removed the bias valve when I put in my big red brakes. I have a C4 master cylinder. Works great with this configuration.

 

Guys, I don't think you can just drop a 993 master cylinder in as it has a different pipe/thread size!

But.... I remember reading something about using BMW E30 fittings to step down or up to the 993 master cylinder.

 

What are the benefits of a 993 master?

 

Waiting for Bill Verburg to chime in. He has the most knowledge on this. I used his advice when upgrading my brakes. Although the stock C4 master is sufficient for the big red brakes, I believe the 993 master cylinder delivers a higher pressure. And yes, it wouldn't be a simple swap the threads are bigger. But I would still do it. The cost of the old 964 master may be more than the 993, iirc.

 

Thank-you everyone. Here's my current thinking, please weigh in with your thoughts.

I will replace the small, early model year, 964-355-025-52, part number master cylinder with a new more common part # 964.355.903.00. That will give me an upgrade from the 20.6mm bore to a 23mm bore. Going the 993 route is a great option but more than I need, especially given the extra steps required to make the parts work in the car.

My final question to all of you;
Will part # 964.355.903.00 simply bolt on to the brake booster in place of the older 964-355-025-52 part that’s in the car without any modifications?

And some have suggested that I have the the bias valve removed at the same time… thoughts on this?

Thank-you for all your help & experience.