Load Sensing Proportioning Valve

[BigD1]

Going over the brake system on my recently purchased 92 Toyota Pickup 2wd, and I found that the load sensing proportioning valve assembly is weeping, so it's only going to get worse. Thought about doing a bypass on it, but I am going to buy the Toyota part, and fix it right. That part is ridiculously expensive, and it cost me $173 total. Supposedly, they can't be rebuilt.

Read over a service manual for a 93 Pickup like my 92, and it goes into detail about using two pressure gauges, and all that mess. But it does say that the pressure can be set to the rear brakes initially, and be close by measuring the distance between shackle and the spring. Mine would be 3.07 inches or 78 mm.

Here is what I am not clear about, and this is my first time around with one of these valves.

(b) Position the valve body so that the valve piston lightly contacts the load sensing spring. (c) Tighten the valve body mounting nuts.

I think I understand what it means, but there is dust boot that covers the valve piston, so I am guessing that the boot has to be pulled down out of the way, so I can see the valve piston.

Anyway, let me know what you guys think about all this mess I got myself into again!

 

[mk378]

Moving the lever by hand, you should feel when it first picks up the piston inside the valve. This is the position it should be in when the truck bed is empty. When the truck is loaded, it pushes harder on the piston which increases the pressure allowed to the rear brakes.

 

[BigD1]

This is just to set it up initially before the brakes are bled.

I am having a hard time getting a picture in my head how this valve can be slid up or down to adjust pressure. Once I get it apart, I should be able to figure it out. The service manual does say that pressure to the rear brakes can be adjusted by sliding the valve up to raise pressure, and to slide the valve down to lower pressure. Just looking at the valve mounted on the truck now, I would have never know that without reading a manual.

 

[andyd]

The whole purpose is to prevent the rear drums from locking. My 80's AMC Jeeps used a Ford part that restricted the pressure somehow. Nothing as fancy as a load sensor spring. though. On Jeeps we called it a proportioning valve. I had 5 Jeeps over 20 yrs and it was bullet proof and DIDN'T leak I would try to reseat the piston and try to bleed the brakes. They were wicked expensive parts, but usually not the problem.

 

[mk378]

Originally Posted By: BigD1
I am having a hard time getting a picture in my head how this valve can be slid up or down to adjust pressure.

The whole principle is that when there is nothing in the back of the truck, you want minimal braking effect on the back wheels so they don't skid. When there is weight on the back wheels, the brakes can work harder without the tires skidding.

So, the valve is designed to respond to the the vertical position of the back axle, which is proportional to the weight in the bed. When loaded, the leaf springs compress and the frame of the truck moves down toward the axle, or the other way to look at it is that the axle comes up relative to the frame. Moving the valve up and down on the frame changes the initial valve position to axle position relationship, thus the amount of braking that will occur with a given weight in the bed.

 

[BigD1]

I think I understand how it all works now after crawling up under the truck again. Spraying it all down with liquid wrench, and letting it soak for a few days before the parts come in.

That valve does have a bleeder screw, and I can't find a bleeding sequence that includes the valve, so at what point would that valve need to be bled? Valve is located on the rear right side, so I was thinking to start off bleeding the brakes on the rear right, and then bleed the valve, and then bleed the rear left.

 

[Jetronic]

Bleed the valve first or you risk pushing air towards the rear right again. Have someone in the vehicle to push the brake while you undo the screw, of course.

 

[BigD1]

OK will do.

I am going to have to buy a brake pressure gauge just to make sure I have this valve adjusted correctly. I think I can get the pressure set initially and be close, but without buying a gauge I will never know for sure.

I wonder when these trucks were rolling off the assembly line if brake pressure gauges were used, or they just used the initial set procedure of measuring load sensing spring to shackle.

 

[Jetronic]

I wouldn't use a gauge per sé, but do some trial brake runs with an empty bed. Start with the valve fairly open, and keep increasing brake pressure as you're slowing down. if the rears lock up before the fronts do, put a bit less spring pressure on the valve.

You'll be surprised how much difference it makes to your brakes if you get enough brake effort from the rears. The factory recommendations are in my experience never the best. Since you first figured to bypass the valve, there's no harm in trying out a few settings.

 

[BigD1]

Got that valve installed today. Always at least one brake line that has to be a PITA to get started.

I got the rubber boot off that covers the valve, and it was nothing but a big pile of brake fluid moistened rust inside of the boot. Piston was corroded really good. Running without a boot covering the valve right now until I figure out something I can use to wrap around it, and then zip tie it. Original rubber boot just fell apart when I tried to get it off the valve.

Loving the Harbor Freight pneumatic brake bleeder I bought recently. Used it twice now, and it makes bleeding brakes super easy, and I can do it by myself with the help of my portable compressor.