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Proportioning valve delete

Discussion in 'The Hokey Ass Message Board' started by Essex_29, Feb 10, 2010.

  1. farna
    Joined: Jul 8, 2005
    Posts: 1,282

    farna
    Member

    "Servo-action" is indeed the "twisting" of the brake shoes inside the drum, which increases the pressure on the shoes against the drum. "Non-servo" brakes don't have this twisting/force amplifying effect and require more pressure and are less effective. AMC used Bendix brakes and that's pretty much the description in the factory service manuals. The terms "Duo-Servo" and "Uni-Servo" are registered Bendix trademarks and why the Bendix manuals use them. AMC also used Wagner brakes for a few years -- Wagners on the American and Classic six cylinder cars, Bendix on the V-8 Classic and Ambassador. Wagner called their servo-action brakes "Compound Shoe", Bendix "Duo-Servo". Both are identical in operation. The Wagners have a different self-adjuster design with rods instead of a cable, but that's the only real difference in the design of them vs. the Bendix. Both work the same way.

    The proportioning valve only limits maximum pressure to the rear wheels (or to a front wheel). Late 60s- early 70s AMCs used two different valves -- limited to 200 psi in the small cars (Javelin, AMX, American, Hornet) and 400 psi in the big ones (Classic, Rebel, Matador, Ambassador). Pressure is limited to that amount. Don't know what the GM combination valve pressures were, but they were used on later AMCs (mid 70s+). They are a fixed pressure also. The idea is to limit rear braking power so that the rear brakes don't lock up in a panic stop. I usually go out on a hard dirt road (plenty of those around where I live!) and slam on the brakes hard at about 45 mph. If the rears lock they are getting too much pressure. Adjust down and try again until they just start to lock. A wet parking lot will work too.

    Speaking of servo/non-servo brakes, AMC used non-servo brakes instead of a proportioning valve in their first front disc brake cars to control rear brake "pressure". Bendix "Uni-Servo" brakes were used in the rear of all 65-67 AMCs with disc brakes (also Bendix -- four piston). Proportioning valves were used with 68-70 Bendix single piston discs. The GM style combination valve was used from 71 on. A combination valve has proportioning, brake light, and sometimes metering functions built into one junction block -- hence the name "combination valve". A metering valve, by the way, won't let ANY pressure to the FRONT brakes until ~100 psi is built up in the lines. It takes ~100 psi to overcome the springs in drum brakes. A metering valve makes sure all four brakes start to engage at the same time. It was used in many mid 70s cars for several years, then fell out of use by the late 70s. Someone figured out it really didn't make any difference if all four came on at the same time, you just can't have the rears locking up before the front or you could find your rear bumper catching up to and/or passing your front!
     
  2. HemiRambler
    Joined: Aug 26, 2005
    Posts: 4,208

    HemiRambler
    Member

    farna, What you described is a Pressure Limiting Valve - and yes it was used on some cars - it may have even been referred to as a "proportioning valve" as it performs a similar function. However it is not technically a Proportioning Valve it IS DIFFERENT.

    In a Proportioning valve the "knee point" is where the pressure relationship changes from say 1 to 1 to then letting the rears only get a PERCENTAGE of the fronts. So the PRESSURE RISE in the REARS (after the "knee point") becomes a FRACTION of the pressure rise at the fronts. SO After the Changeover Pressure is reached the rear pressure does continue to rise - just not as fast as the fronts - hence the term "porportion"

    The graph I was earlier thinking about was a Kelsey Hayes graph very much like the one pictured below. I got one with my Direct Connection Adjustable Proportioning valve (I bought 20 years ago) which I believe was actually manufactured by Kelsey Hayes. The ADJUSTMENT is varying the KNEE POINT (the pressure at which the portioning ratio change occurs) by twisting the knob you are changing the pressure - just not necessarily limiting it in a "flat line" sense - see pics.

    Now whether or not the aftermarket offers Pressure Limiting valves as a common practice - I dunno - the only ones I ever bought were actual Proportioning Valves.

    BTW - Notice the middle picture - that was for a Proportioning Valve with 5 preset adjustments. Depending on which adjustment you chose the pressure curve would follow along until the changeover pressure was reached and then decrease appropriately.

    Last Pic (on right) shows the pressure relationship with a Pressure Limiting Valve as you described.

     

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    Last edited: Feb 23, 2010
  3. farna
    Joined: Jul 8, 2005
    Posts: 1,282

    farna
    Member

    Hmmm... well, I think I misunderstood the way the valve works. The "knee point" then is 200 psi and 400 psi on the AMC valves (which I believe were made by Bendix) is what you are saying, and from your explanation and the tech literature you provided, I'm sure I was mistaken and you are 100% correct. Makes more sense than just limiting overall pressure too.

    The AMC tech manual says "Both the front and rear brakes receive full master cylinder pressure up to 200 psi. At higher pressures, the proportioning valve regulates the pressure to the rear wheel brake units." I was just reading it wrong. That is in the 1968 manual, later ones add the 400 psi for the larger cars.

    Thanks for the correction!!
     
  4. Good Christ, man! How many times does it need to be said?: DO NOT TAKE CHANCES ON BRAKES. Make them work before you get on the road. I do not recommend running without a prop valve. What has worked by chance for some may not work for you. Do not play games with a combination valve. Use an adjustable unit or leave the combo valve alone.
     
  5. B Blue
    Joined: Jul 30, 2009
    Posts: 281

    B Blue
    Member

    I removed my proportioning valve because the rear brakes do not lock up before the fronts. Where do you recommend I place the proportioning valve? I favor the center console as the glove box tends to get a little messy, what with the registration and insurance papers.

    Bill
     
  6. farna
    Joined: Jul 8, 2005
    Posts: 1,282

    farna
    Member

    The valve can be mounted under the car in the rear or coming up through the trunk. Once it's set right you really don't need to touch it again. That's why a lot of rodders use the factory type non-adjustable combo valve right off the master cylinder. More on that below...

    There's not a thing wrong with using a combination valve (factory proportioning valve) instead of an adjustable valve on most cars. I wouldn't use the factory type valve on anything real light like a 32, but get into the "fat fender" cars that weigh 2500 pounds or more and you're about the same weight as the cars the factory valves came in (Mustang, Camaro, and larger cars). For something that weighs less than 2500 pounds you may need a lower "knee point" to prevent the rears from locking up. See www.mpbrakes.com and read their technical pages. They have been recommending the GM combination valve for most street rods for years. Only something real light would need more.

    I agree with you -- unless you have the same type brakes all the way around (four wheel drums or four wheel discs) you need some type of proportioning valve. With four wheel discs you may need them too, unless it's a factory type rear disc setup. They are usually designed not to need a proportioning valve. When both ends have balanced brakes of the same type a proportioning valve usually isn't necessary or used -- that's why the old drum/drum systems don't use one. The trick is that the rear drum size and wheel cylinder size are matched to work properly with the fronts though -- same with four wheel discs.

    So what happens without a proportioning valve? In normal driving under normal (dry) conditions -- nothing. Brakes work fine. The problem is when conditions AREN'T normal -- mainly when the road is slippery (wet, snow, ice, loose gravel or sand on road, etc.). In normal driving you would usually be okay, you could feel or hear the rear brakes lock and given room and time react before anything bad happened. "Given room and time". In a panic situation -- some kid follows a ball in front of your car -- you don't have "room and time", and the rears lock and you slide. Sliding isn't stopping. Then there's the slippery conditions. You hit the brakes hard going down a wet/snowy/icy incline. You were going a little too fast (never!!), someone slammed on brakes in front of you, and suddenly your rear end wants to pas the front. Such situations are what the proportioning valve is really for. You can't control your driving conditions or other drivers. Many on this board are good enough drivers to react reasonably well to panic situations and control a car even if the rear brakes start to lock. But it's hard to stay off the brakes when you know if you press harder they will lock and make the situation worse when it looks like you're about to hit something and wreck your precious rod (we'll assume it's not your fault here).

    I've been there on a motorcycle -- hit am unfamiliar blind curve a bit fast then discovered the curve got TIGHTER as I went around. You can't slow on a bike in a turn -- the bike wants to stand up and go straight if you do. You have to leave the throttle where it is or accelerate to pull around a curve, braking can kill you! But stepping on the brakes probably would have put me through the trees. So I forced myself to look around the curve where I needed to go (something else that's a must on a bike!) instead of the trees and side of the road I was afraid I might hit, and stayed on the throttle. Rode the white line on around it, but got around it! Then I slowed the f--- down, especially in curves I couldn't see good around!!
     

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