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914World.com _ 914World Garage _ Proportioning valve

Posted by: Socalandy Jul 2 2013, 04:25 PM

Does the proportioning valve work as a pressure restriction device or is it tilt/pitch activated like a VW Vanagon? Based on how its mounted my guess would be pressure.

Thanks in advance!!

Posted by: GeorgeRud Jul 2 2013, 04:28 PM

Pressure

Posted by: r_towle Jul 2 2013, 04:34 PM

It's not a pro portioning valve in the classic sense of how they operate.
If you search here, Eric Shea did a very good explaination of it, and I believe Chris Foley did some reverse engineering on it as we'll.

Both great reads.

Posted by: 914_teener Jul 2 2013, 04:43 PM

Andy,

Yes...Rich is right....Eric did a really good write up thread of how it works......

It is a pressure valve technically.

Posted by: Jeff Bowlsby Jul 2 2013, 06:04 PM

The factory manual has details too.

Posted by: Steve Jul 2 2013, 07:28 PM

Just toss it and put in a "T"

Just kidding!!!

Posted by: toolguy Jul 2 2013, 08:24 PM

Very basically, it has a movable piston that is backed up by a spring, , and the outside adjusting screw and locknut place pressure on the back of the piston via the spring. . . . the front side is connected to the incoming brake line. . as you apply brake line pressure, the piston moves inside and when the preset spring pressure is overcome, the piston moves far enough
to cover the ports to the rear brake lines, restriction any more rear pressure.
IIRC, Reduce spring pressure[turn the adjuster out] and you'll get less rear brake. . Increase the spring pressure and you get higher rear braking. .
Warning. . by increasing the rear brake, under extreme hard braking, the car will want to lock the wheels and skid / swing the rear end around when braking in a curve. . even in a straight line, when braking hard, the rear wheels will tend to lock up as weight is transferred to the front wheels. . .