Porportioning Valve

[FMTRVT]

Didn’t mean to dissuade you, just do it well.

If you have RABS, then you won’t have the lever. It’s only with the mechanical prop valve that adjusts pressure to the load (spring sag).

The lever style adjustable prop is good. I’ve used the knob version and the knob needs to be marked and when you forget your place, turn it all the way and back out the number of turns to where you want it.

As you thought out, the best thing after installing it is to go out and see where the lever needs to be under different load conditions. You don’t have to get nuts, just pick unloaded, full weight and ½ load. If you want to spend the time, get it scaled (if you don’t already for carrying deliveries) so you know the unloaded and GVW weights, then sometime get the ½ load.

As a driver it can be hard to know if the wheel skid occurs from the front or back. The best way to handle this is to put a piece of duct tape on each wheel, then have an observer watch from the side of the road to confirm wheel skid. With duals, it may be easy to tell, though.

The temperature of the brakes has an effect on how well they work and are balanced front to rear. Cold drum brakes don’t do too much. The best way to warm them up for consistency is to make about 15 stops from 40 mph in one mile intervals. That’s the same prep my drivers would go though in preparing to run their tests. Each time you make a “test” stop, you need to cool down the brakes for the run attempt. During our tests, the vehicles usually needed about 5 minutes between each stop to return to 200°F, where we did our thing. But that was with a full 60 to 0 mph stop. I would recommend just doing this from about 40 mph and getting off the pedal as soon as you know the skid tendency. Without ABS, we would flat spot tires pretty quick. The 200°F temp is also about where most vehicle brakes are in normal driving.

You could also get real nuts and take a trick from our goofy stuff. You can mount a ¼ in rod to the rear axle that stands up next to the frame, with some clearance to the bed when it’s at full load and figuring some “bump” clearance. You can mark the frame where the top of the rod is under light, ½ and GVW loads. This way you’ll know what setting your lever should be at without guessing how much load is in the bed.

While duel rear wheels are needed to carry the loads, they do make for interesting driving when the vehicle is unloaded and the road is wet. Each dual tire is at ½ the tire loading of an SRW vehicle, so with the lighter pounds per square inch loading, it skids easier. A couple of trial runs in wet weather may be appropriate, but you need to be very aware as getting out of control with a skid happy rear axle is pretty easy, too.

Despite all of the talk of testing for the best setting, you could develop a feel for the setting just in normal driving.

[WhiteGardens]

Hey, thanks again.

You haven't deterred me from trying to use a prop valve.

Luckily I can see the tires from my rear-view mirror, so it shouldn't be too hard to figure if I'm skidding or not.

So what are your thoughts on a GM style prop valve that "adjusts" itself under braking conditions ???

[FMTRVT]

That's how all prop valves operate. They proportion the rear pressure compared to the front brake pressure. Like the example I gave, at 300 psi (front and rear) they start to proportion the rear pressure at a slope of 43%. Up to the prop's internal valving, the pressure to it is always the same as the front pressure.

[WhiteGardens]

Originally Posted by FMTRVT

That's how all prop valves operate. They proportion the rear pressure compared to the front brake pressure. Like the example I gave, at 300 psi (front and rear) they start to proportion the rear pressure at a slope of 43%. Up to the prop's internal valving, the pressure to it is always the same as the front pressure.

OK, so even the adjustable ones work on a slope if I'm interpreting you correctly.

If that's the case then I feel better.