Brake Proportioning Valve Notes

by: Bill Gregory (sansho@rennlist.com)

Date: August 9, 2002

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 Proportioning valves are used to limit brake pressure to the rear brakes when there is a danger they may lock up before the front brakes under heavy braking, causing severe handling problems.

One of the early uses of a proportioning valve on a 911 was in 1984, when the piston size on the rear brakes was upgraded to be the same size as the front.  Shortly before that a proportioning valve was also used on the 930 Turbo.

On non-Turbo 964's, Porsche uses two different size proportioning valves, which are measured in bars: the 90-91 C2 has a 45 bar valve, and all other C2 and all C4's have a 55 bar valve. Sometimes, people who take their 964's to the track will upgrade to the 964 Turbo 60 bar valve, to put more braking into the rear and balance out the braking on the track. Interesting to note that the Porsche Cup Racing 964's didn't have any proportioning valve, but they also had some different components in their braking system, suspension, and drivetrain.