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964 brake upgrade and master cylinder

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.
Old 05-14-2019, 09:55 PM
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Quote:
Originally Posted by Grants911 View Post
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 993 m/c upgrade is only for 964 C2 which have a different vacuum powered brake booster.

The electro-hyd system used on you car is basically the same in the C4, 964t and 993t and 993C4

There were 2 different m/c used on these systems
23mm on C4, t & TL
25.4mm on RS

If your system was properly upgraded w/ both fronts from a 928GTS and both rears from an RS or base 993 then I'd use the larger 25.4mm RS m/c 964 355 904 80 though the stock 23 also works fine 964.355.903.00

There is either some confusion or the early cars may have used a 20mm m/c, I've never seen one but some of the factory docs do list a 20.4 for C4

The RS booster also had a higher boost ratio but that's not a big issue for the purposes of this upgrade.

993 355 903 00 is the 993 23.8mm m/c that is compatible w/ your system and is also a bolt on. the 993RS used the same 25.4mm 964 m/c part as the 964RS
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Old 05-15-2019, 04:09 AM
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Thank you for your excellent reply. I will likely stick with the stock c4 m.c.

It's still a mystery to me what the existing part number is on my car... my car was built in Dec 1989. It looks from the "bolt paint" like this was an original factory part; an ATE product 964.355.025.52. Does anyone have any thoughts?
Old 05-15-2019, 07:28 AM
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I can only find this part as a used part, it is not available new and it is not listed in the PET catalogue.

I assume that part number 964.355.903.00 will work fine in my car regardless?
Old 05-15-2019, 07:34 AM
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Originally Posted by Grants911 View Post
I can only find this part as a used part, it is not available new and it is not listed in the PET catalogue.

I assume that part number 964.355.903.00 will work fine in my car regardless?
Quote:
I assume that part number 964.355.903.00 will work fine in my car regardless?
yes

As I said I think that some of the very early 964s may have used the smaller 20.6mm m/c which is probably what you have, later ones and the turbos used the .00 which is 23.8 and then the RS used the 25.4
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Old 05-15-2019, 11:12 AM
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964-355-025-52 is a part number superseded early on .
Suspect the ussual ploy of replacing the 1st 3 digits from 911 to 964 to claim “99% new car” ...
I can get you one if you need originality ?
Old 05-15-2019, 05:40 PM
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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.
Old 05-16-2019, 09:27 AM
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Originally Posted by Grants911 View Post
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.
Again you confusing the vacuum boost and the electro hydraulic boost, possibly because of poor advice elsewhere.

Your C4 electro hydraulic boost is basically the same for all 964C4, 964t, 964RS 993C4, 993RS, 993t

there were different sized m/c and different boost ratios but the systems are all mix and match.

in the regular 964 and regular 993 a vacuum boosted system is used, here the 993 and 964 are different and incompatible, you use one or the other w/ no mixing of parts

Yes use the updated 964 part #, that will be fine yes the p/v can be removed w/ ill effect, the factory did that on the Cups, what you have is basically the same as the brakes on a 964RS or 964Cup(assuming that the rear was addresses as well as the fronts)
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Old 05-16-2019, 02:08 PM
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Awesome information Bill. I owe you.

Yes, the rears were also done... having the equivalent of the 964RS brakes is more than I need. All great info and news.

Thank-you to all those who helped me sort this out.

Grant
Old 05-16-2019, 04:42 PM
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