Quote:
Originally Posted by Grants911
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.
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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