# adjustable proportioning valves



## testvwpilot (Jun 5, 2004)

how do i


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## testvwpilot (Jun 5, 2004)

*Re: adjustable proportioning valves (testvwpilot)*

how do i run rear brake lines from master cylinder to a single adjustable proportioning valve then to rear calipers 92 16v gti no abs


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## GTIspirit (Dec 13, 2002)

*Re: adjustable proportioning valves (testvwpilot)*


_Quote, originally posted by *testvwpilot* »_how do i run rear brake lines from master cylinder to a _single adjustable proportioning valve_ then to rear calipers 92 16v gti no abs 

I'm not sure if this is such a good idea, I've thought it out with friends as follows and would like to know what other people think.
VWs, at least A1 and A2, have dual diagonal brake circuits. So the left front and right rear, and right front and left rear are tied together. This keeps the diagonal circuits independent so in the unlikely event of loss of brake pressure to one of those you can still stop the car. If you T the rear lines together the dual diagonal circuits are no longer independent. So loss of brake fluid pressure to one corner would affect the remaining three corners and make it hard to stop.
So what my friends did for racing was to run two prop valves to the rear to keep the dual diagonal circuits independent. With this setup you can even adjust left to right bias, in addition to front to rear. But I don't want to fiddle with that and will be trying a brake pressure regulator, in lieu of a prop valve. This essentially does the same thing, keeping left to right split even, but I'm not sure if it will have the range of adjustment that a true prop valve has.








Courtesy of Carmadillo, who has these brake pressure regulators at a really good price, just a hair more than a single aftermarket prop valve.










_Modified by GTIspirit at 1:50 PM 6-10-2004_


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## Racer_X (Jul 14, 2002)

*Re: adjustable proportioning valves (testvwpilot)*


_Quote, originally posted by *testvwpilot* »_how do i run rear brake lines from master cylinder to a single adjustable proportioning valve then to rear calipers 92 16v gti no abs 

The best way is to get a Tilton dual master cylinder setup and run one master cylinder for the front brakes and one master cylinder for the rear brakes. The dual master cylinder setup has a bias adjuster for the rod that connects between the master cylinders to the brake pedal. 
Another alternative is to get a dual circuit master cylinder from a car that's designed with a front/rear dual circuit braking system. Plug one of the rear lines and run the other through the adjustable prop valve and to a "T" connector at the rear axle area to split into the two rear brake lines. 
The problem is that VW braking systems are designed to be a dual diagonal system. The rear brakes are on separate hydraulic circuits. The VW Master cylinder doesn't have enough capacity to run two front brake calipers on the same hydraulic circuit. If you try to hook it up that way, you'll bottom out the front half of the master cylinder before you get much braking force in the front brakes, then the rest of the pedal travel will be rear brakes only. This will make it seem like the brakes are horribly overbiased to the rear. But even a good prop valve won't fix the situation because the front circuit will never gain pressure after the piston bottoms out in the master cylinder bore. The problem isn't too much rear brakes, the problem is not enough front brakes and a prop valve won't fix that.
One "solution" that is not recommend (illegal in most of the US and potentially very dangerous) is to leave the dual diagonal plumbing and use two "T" connectors in the rear lines, one to join them to run to an adjustable prop valve, and a second to split them back to two separate rear lines to go to the calipers. That turns your brake hydraulics into a single circuit system. That's been illegal in the USA for cars made since the mid 1960's. If one caliper or line fails, you'll lose brakes to all 4 wheels and be unable to make a controlled stop. This is not good.


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## GTIspirit (Dec 13, 2002)

*Re: adjustable proportioning valves (Racer_X)*


_Quote, originally posted by *Racer_X* »_
One "solution" that is not recommend (illegal in most of the US and potentially very dangerous) is to leave the dual diagonal plumbing and use two "T" connectors in the rear lines, one to join them to run to an adjustable prop valve, and a second to split them back to two separate rear lines to go to the calipers. That turns your brake hydraulics into a single circuit system. That's been illegal in the USA for cars made since the mid 1960's. If one caliper or line fails, you'll lose brakes to all 4 wheels and be unable to make a controlled stop. This is not good.

That's precisely why I recommended to use two separate proportioning valves or the single dual one pictured above.







Whether it's useful or not, there is the wow factor in saying that not only do you have a prop valve to adjust brake bias, but that you can also adjust side to side brake bias in addition to front rear bias.


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