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Brake Master Cylinder Design in Bosch 5.7 ABS/Stability System

Brake Master Cylinder Design in Bosch 5.7 ABS/Stability System

Brake Master Cylinder Design in Bosch 5.7 ABS/Stability System

(OP)
Question for any Bosch 5.7 experts out there... What is different/unique about master cylinder design in the Bosch 5.7 system that has an ABS pump in the hydraulic block PLUS a separate boost (pre-charge) pump (for stability management)?

Some background... I have a 2001 Porsche 996 Turbo which has the OEM Bosch 5.7 ABS/PSM system. (PSM = Porsche Stability Mgmt) The brake pedal feel is soft and pedal travel is longer than desired. Stock MC piston is 25.4mm. As recognition of this issue, Porsche increased the MC piston size to 27mm in the next generation, the 997. However, Porsche did not use the same ABS system in the 997 (eliminated the separate boost pump).

I installed the 997 MC in my 996. The pedal feel was much improved, but the ABS/PSM system fails to pass readiness tests and is disabled. I'm not a Bosch 5.7 expert, but I've done some digging and come to the working theory that the 997 MC isn't compatible with the Bosch 5.7 system because it does not allow the boost pump to build brake pressure when the brake pedal is not depressed.

My suspicion is the 996/Bosch 5.7 compatible MC has a non-typical bypass circuit. Does this sound right? Is this a well known issue?

Anyway... the point of all this... How do I get a MC with a 27mm piston that is Bosch 5.7 compatible?

Thanks for any insights!

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