I am installing a frame mounted Wilwood Tandem Master Cylinder with remote reservoirs along with a Combination Valve Kit (proportioning valve) from Master Power Brakes. The reservoirs will mount higher up in the engine compartment. Common practice is to mount the combination valve so that it sits below the master cylinder. In my installation, the best location for the combination valve will be a bit above the master cylinder but well below the reservoirs. The instructions with the combination valve say the valve should be below the ports on the master cylinder. When I asked the Tech Team at MPB they said "with the reservoirs being higher, one could make the argument that it doesn't matter". Not a really a clear or definite answer. Does anyone have any first hand experience with combination or proportioning valves mounted above the master cylinder?
Describe your brake system. If you are not going to connect the combo valve's pressure differential switch to a warning light, why install it? If you need proportioning, why not just use a much better adjustable prop valve down stream from the rear brake m/c port instead of the fixed value combo valve?
I think what they (WILWOOD) is saying is that the valve(s) should be mounted below the MC out-port center line as there is a possibility of fluid drain-back if any valving is higher than the MC out-port(s). Same theory as using a #2 RPV on an application that has the MC below the front/rear disc center line. Why a COMBINATION VALVE? Is it one of MPB 'universal' valves? (IMO), it would be best to use a free-standing front metering valve on the front circuit (if DISC/DRUM) and an ADJ bias valve (similar to an OEM PPV) on the rear circuit. An actual OEM PROPORTIONING VALVE is calibrated for a certain vehicle/brake size application. There are ADJ PPV available, but $$$. I would be afraid of a 'universal' valve. With an ADJ BIAS VALVE, be certain you follow directions closely as to calibration.
***************** - PROPORTIONING VALVE SELECTION- Selecting the correct adjustable proportioning valve for your vehicle entails not only selecting the proper point at which slope limiting begins (the knee-point), but also selecting the proper rate at which rear brake line pressure builds after that point (the slope). Nearly every adjustable proportioning valve on the market today has an adjustable knee point (the point at which the rear brake line pressure begins to be controlled), but a fixed slope (the rate at which it builds beyond the knee point). While one parameter is adjustable, both are critical to system performance. CENTRIC BRAKES -
As mentioned above a combination valve is calculated to a certain brake system, there is no “one fits all” combination valve. I purchased a master cylinder and combination valve from Master Power Brakes, it was installed in a completely new build using all new Baer brakes. When the MC and valve were purchased Master Power Brakes was informed of the brakes being used. After installation the car would not stop no matter how the system was plumbed or bleed. After much frustration the combo valve went in the trash and a Kelsey Hayes adjustable valve was installed, wow! The car now stopped! In my opinion there is no combination valve that will work on a non factory brake setup unless the valve was purposely designed (by a computer) for the application.
Mounting location I don't think is such a important part but I would definitely mount it below the master cylinder. From the factory I have seen them mounted close to the bottom of the master cylinder to being mounted way below the master cylinder to the side of the frame rail so anywhere from about 3" of brake line to 16" of brake line between master cylinder and valve. I wouldn't want anything mounted above the master cylinder if possible. The fact that fluid runs downhill is a given, so the fluid in the valve would always be wanting to drain back into the master cylinder (as seen in under floorboard masters). Also too it's just one more place to trap air bubbles and spend enormous amount of time trying to bleed to get any air out of. .
Wilwood makes a dual master cylinder ***embly that is MECHANICALLY bias adjustable. No need for anything else. It can even be modified to be adjustable while driving for racing use.
A big Thank You to all for the input and good advise on the proportioning valve placement. My conclusion is that it would be best to forget the MPB Combination Valve for this frame mounted master cylinder application and go with an adjustable proportioning valve in the rear brake line for all the reasons described. The plumbing will be significantly less involved as well which is a bonus. I appreciate the experience and willingness to share that experience here on the H.A.M.B. Your hands on experience along with your litany of mistakes and successes is far more valuable than some instruction sheets. SanoDano out.
I agree with ditching the combination valve and then using an adjustable on the rear. You might not even need the rear adjustable valve, you can tweak rear brakes with wheel cylinder diameter. But the adjustable valve does offer more adjustment.
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