Proportioning Valves - Myth or Reality??

There ARE some vehicles that wear front and rear brakes at ROUGHLY the same rate...they participate in a series called Formula 1. The rest of us have to deal with the realities of physics, damn pesky weight transfer and whatnot. In nearly 100% of roadgoing vehicles, except for a one ton truck carrying 3000lbs of gravel in the bed, it is impossible to apply full brake force to the rear, because given weight transfer of a moderate to spirited stop, you've got about 80% or so of vehicle weight transferred to the front wheels. If one were to apply an equal braking force to all four wheels, what's gonna happen? You'll drag the locked up rear tires with you as you slide off the road......

ABS?? Has nada to do with the function of a proportioning valve, and serves no purpose in this discussion. ABS dumps 100% of line pressure to a given wheel once that wheel speed is sensed to be reducing more rapidly than the others.

 

Been there done that too-fully sideways in a Tbucket with no 'belts or 'bars under braking is scary as hell. went home got on the phone and ordered up one of them Wilwood adjustable prop valves thru it in the rear line and proceeded to adjust it until I could feel that the rear end was not going to step out any more under braking.

 

The only vehicles I have ever seen those on were mini import trucks--and they worked weird. when you picked the vehicle up on the lift, as the rear tires came off the ground, they locked up! I think they are more like mini "emergency master cylinders" than prop valves.
think of an airborne pickup like an arrow. you really want the rear end to stay behind you. and if the rear tires are locked (while moving in a straight line) in theory, the drag will help "plant" the vehicle.
of course by then Grandma will have turned the wheel one way or another causing a rollover and many weeks in the hospital...

 

C9, This big/little tire thing has a certain logic that's tough to argue with - I suppose the CHALLENGE would be to determine whether or not you fall into this category. If your rears never lock up - I suppose that's a clue your FRONT BIASED way too much. Keep in mind there are other ways to shift your bias from front to rear and vice versa.

The rears do lock up.
About the same time the fronts do far as I can tell.


Ahmmm....I don't think you can draw this conclusion just yet. Actually if you delve further into a Proportioning valve you will see they are designed such that they actually CHANGE thier rate from basically NOT being there (initially) to then proportionately reducing your rear line pressure. Changing the adjustment CHANGES the POINT of pressure in which this relationship occurs. The RATE of proportioning is determined by the spring - the point at which it enters the equation is determined by the preload in the spring. So while you might have removed all the external preload - the spring was still there and still able to function - meaning you didn't defeat it by adjusting it - you merely changed the pressure where it activated.

True, but eventually the pressure at the rear drums will be the same as the pressure at the front discs so what the propo valve is doing is delaying the onset of max pressure to the rear drums.



Somewhere in there, someone mentioned diagonal braking with a failure in one side of the system.
I believe that's a Volvo thing and I don't think they do it anymore.
I'm guessing that many drivers locked up one front wheel, the car took a trun toward the locked up front wheel and with what weight shift there is, lost control.
Better to have the brake systems split front and rear than diagonally imho.


One thing I do find interesting is from my dirt bike days.
Go down a hill with the rear wheel locked and it wouldn't be long until the rear of the bike was trying to p*** you.
Doesn't seem right, but that's the way it worked in that situation.

Locking up the rear wheel only on the flat at speed brought the same situation into play although not as bad as going downhill did.

Leave the rear brake locked up long enough - specially on the flats - you'd get sideways, eventually the sliding bike would hit something that caused it to go upright (or the rider would let off the brake after getting deep into the slide) and launch you off the high side.

I can attest to the fact that landing on your feet and trying to run 40 mph after doing the high side bit doesn't work.

Well . . . actually it does, but not for long....

 

ok 2 things that rear brake valve was used on big trucks also my son has a 82"i think" gmc dump truck ch***is that has one of those mounted above the rear end.this truck was bought new by the city of Worcester M*** and was used for plowing/sanding it is the same truck as my Chevy c30 but the gmc model.

Next how does one reset the proportioning valve once it goes "over center" after a break line goes out? I have an s10 blazer I replaced every brake line in.I did this because it was used to haul a boat and got backed in the water a lot by the guy I got it from .Didn't want any problems from rusty lines well the prop valve lost its set and no matter what i,I can't get it reset.The manuals I have don't cover resetting it.I have tried clamping it back using a C clamp and I use vacuum pump to do the bleading.now I worked 5 years straight as a brake and tire man for car dealership but that was long before abs systems and prop valves

 

tap your master cylinder gently with a hammer. then do the same to the prop valve. if it does not work, the thing is just plain broke.

 

Stickey ~ Good Choice on ditching the extra inline pieces. I have the same MC on my 52 pu, the neat thing about these MC's is they have the Residual valves built in and if you have Disks on the front you need 2 lbs residual pressure there and 10 lbs on the rear. To make sure you have the correct 69 Mustang MC ~ take a toothpick and stick in each of the output fittings, if there is a little resistance you have the correct cyl with the residual valves.

 

If your Blazer has a typical Gm combo valve such as this, remove the rubber cap shown on the left side and push the pin back in flush which will restore the correct setting. You can also remove the switch and verify the 'V' notch is centered under the opening.

CC

 

Ahmmm....I don't think you can draw this conclusion just yet. Actually if you delve further into a Proportioning valve you will see they are designed such that they actually CHANGE thier rate from basically NOT being there (initially) to then proportionately reducing your rear line pressure. Changing the adjustment CHANGES the POINT of pressure in which this relationship occurs. The RATE of proportioning is determined by the spring - the point at which it enters the equation is determined by the preload in the spring. So while you might have removed all the external preload - the spring was still there and still able to function - meaning you didn't defeat it by adjusting it - you merely changed the pressure where it activated.

C9 - No, They (the proportioning valves) don't work that way. The Metering valve works this way - if you had JUST the metering valve (and some were made this way - meaning solely a metering valve ) then YES I agree to your statement. If your proportioning valve was working in this manner - it's defective and needs replaced. The "piston" in the proportioning valve is ALWAYS in the equation - meaning that it reacts to pressure - the reaction is movement - movement means INCREASING your VOLUME on that side - Increased Volume REDUCES PRESSURE. There's no time delay involved to restore FULL pressure. Matter of fact that would work exact opposite of what you want/need. AS you brake - be it HARD or MEDIUM or LOW - you are CONSTANTLY changing your BRAKE REQUIREMENTS - the car's WEIGHT TRANSFER is based on how FAST you are going combined with how HARD you brake. Watching you FRONT END nose dive MORE under HARSH braking is an indication of this (makes sense too) so as you transfer MORE weight (to the front) you RESULT in the FRONT BRAKES being able to carry MORE braking load requiring the REARS to be SUBSTANTIALLY LESS PRESSURE than the fronts. As you apply the brakes under LIGHT braking you transfer very little weight and want the fronts and rears to "see" nearly the SAME pressure - or very little proportioning. Bottom line - the proportioning is 1 to 1 in the very beginning - as you continue to increase the pressure you reach the changeover point where you START to reduce the rear pressure proportionately to the fronts.

The idea that different sized tires affect this equation is a keen observation - but also understand that the tire's ability to carry DIFFERENT amounts of braking load is also VARIABLE we must deal with. An excellent (yet backwards) example of this is how we INCREASE WEIGHT TRANSFER to GAIN TRACTION in a DRAG CAR. A car that SHIFTS more weight to the REAR tires can attain MORE TRACTION than an identical car that does not. It is this SAME REASON that when a car TRANSFERS MORE weight to the FRONT tires - they can then do MORE BRAKING than before. "more braking" means you (me - us) the driver can depress the pedal MORE to STOP Faster - the problem is that as the FRONTS INCREASE their ability to achieve traction the REARS LOOSE some!!! AS this happens we want LESS pressure to the rears to prevent lockup. The "RUB" (pun intended) is that all this **** happens under a **** load of variables!!!!!! Hot (sticky tires), COLD HARD tires - HARD braking - LIGHT braking - it goes on and on and on. So the factory comes up with this nifty idea that tries to work under this ****load of variables - it's only PERFECT under a single set of cir***stances - while everything else is a compromise. So yackity yackity yackity - I knwo what you're thinking - wait are you still awake??? Yes I will get to the point......when are these cir***stances MOST IMPORTANT to you - I will ***ume it is when the ROAD CONDITIONS are the WORST!!!!! So that tells me that your testing should include turning in WET weather conditions as well as DRY.

Now naturally -some of us NEVER drive in wet conditions and therefore may get vastly different results than those of us who do. Some drive on concrete - while others blacktop - still others - a dirt road. Do we really test all these ??? I sure as hell don't - there's not enough days in my life - so I try to copy a factory system - that is close to my car/truck and then try a couple scenarios and see if things work like I think they do.

And if ALL THAT **** DOESN'T WORK - then I am cutting two holes in my floor so I can jam my feet through to the ground. I will wear my "good" boots and by shifting my toes I can generate all the Brake Bias I need. I may bring a spare pair incase I need replace the pads - errh I mean the soles.

Seriously - I find your Dirt Bike anaolgy quite enlightening - physics doesn't know if it was a car or bike - so I believe those are great comparisons to make.

I tried that too - amazingly I found it easier to land hard enough to bounce up and off the bike - but in true dirt biker fashion I NEVER let go of the bars - this of course was a mixed blessing as I couldn't get my feet back on the pegs so I dragged them behind me in a feeble attempt to maintain balance - of course I eventually slowed enough where that wasn't workign either and FLOP - I went over. Lucky for me I protected the exhaust pipe from getting damaged by keeping my leg between it and the ground. Eventually the skin grew back......