Load Sensing Proportioning Valve and Bypass Valve ???

[Bala]

1) What is the function of this LSPV & BV? I'm guessing it determines braking ration between the front and rear.???
2) It looks like it has a bleeder valve. Do you bleed it just like bleeding brakes?



My reason for asking is that I installed new front rotors, calipers, and pads in October of 2009. Until today, I have never done anything with the rear drum brakes. I noticed my pads were already worn and my front rotors get extremely hot!!! They have a little rainbow stripe on them...I'm assuming hard spots...not good. None the less, I installed new pads on straight rotors.

I just finished a rear drum brake overhaul; new wheel cylinders, new shoes, new spring hardware, was able to resurface the drums, and had a good all around blood letting. BTW - those 4WD drums are almost impossible to find (discontinued)...and if you do...be prepared to spend $120 or more each, plus shipping.

I was thinking that my rear brakes weren't doing much, making the fronts do all the work. Check out my old shoes.



However, I was wondering if the LSPV & BV had anything to do with it. Any words of wisdom?

[timsrv]

An empty van does about 80% of it's stopping with the front brakes. If the back brakes got as much pressure as the front you'd be dealing with rear wheel lock-up during a panic stop. This is bad, as it can lead to other problems like fish-tailing or losing control. As physics would have it, when you slam on the brakes, the majority of the weight is shifted to the front wheels, so that's where you need the stopping power. Due to surface area, and the above statement, correctly operating rear brakes on a van should last through about 4 or more sets of pads on the front.

Now if you're carrying a heavy load in the back, then the equation significantly changes. Thus the reason for the LSP & BV (Load Sensing Proportioning & Bypass Valve). The LSP & BV arm that's attached to the rear axle housing monitors the axle's position relative to the body. The movement of this arm allows the valve to control the flow of brake fluid to the rear brakes.

The LSP & BV has a bleeder on it, but I usually ignore it. Sometimes it comes in handy during a difficult bleed, but whenever you bleed the back wheel cylinders the fluid/air also flows through the LSP & BV so bleeding that separately is usually unnecessary.

Your worn out brake shoes are evidence that the LSP & BV is allowing fluid to flow to your rear brakes. Still, it's a good idea to check this from time to time to be sure the flow is correct. An occasional visual inspection is also a good idea. Just make sure the arm is there, it's undamaged, and there are no leaks on or around the LSP & BV. The Toyota service manual outlines a test that includes the use of hydraulic pressure gauges, but unfortunately most of us don't have this tool or the budget to purchase them. So unless you happen to have a set laying around, the next best thing is to perform a "test panic stop". This should be performed on grass or gravel because doing it on dry pavement is a bit hard on you and your equipment. Basically you get the van up to about 30mph then brake hard. The objective is to verify all 4 brakes lock-up, but you want to see the fronts locking up before the backs. It can be difficult to do this test by yourself, so you may want to find a helper to watch from a safe distance, then have him/her report what he observes.

Keep in mind that having too much power at your back brakes is potentially more dangerous than having little or no brakes back there. If the back brakes lock up prematurely (before the fronts) you may find yourself fish-tailing or possibly losing control during a panic stop (not good). Having little or no rear brakes is not good either, but will only slightly increase your stopping distance...........unless of course you are carrying a heavy load, then the back brakes become much more important. Tim

[Bala]

Thanks Tim! If anything, I was more worried about overloaded front brakes. With new brakes in the back, braking (reverse braking esp.) has so far been a whole new experience. This was my first experience with drum brakes. I just hope this will relieve some heat generated on the front rotors. I don't plan on pulling a heavy load in the back...I want to baby my automatic transmission. So no towing or heavy lifting for me. Just pure kick ass van riding!

[timsrv]

One more thing that should be mentioned about the LSP & BV is how it can be influenced by modifying your ride height. If you lift or lower your van, you will also need to make a linear adjustment to the LSP & BV. When I say lift or lower I'm not talking about changing rim or tire size. I'm talking about physically changing your suspension (cutting springs to lower, adding spring spacers to raise, cranking torsions to one extreme or the other, etc). If you do anything that changes your spring rates (add extra leafs, air shocks, air bags, overload devices, etc) things get more complicated. The LSP & BV relies on factory spring rates to correctly adjust flow to match the load. If you add overload devices then it significantly alters performance or even defeats the purpose of the LSP & BV. I have a couple of vans that carry heavy fixed loads (one carries about 2,000 lbs and the other one carries about 3,000 lbs). To correct the ride I have installed overload devices. As a result of altered spring rates my LSP & BV does not sense the load and therefor only allows minimum flow to the rear brakes. Since these vans are already overloaded I need maximum brake power to the rear all the time. To solve this problem I lifted the sense arm to the maximum loaded position and fabricated a bracket to hold it there. Note the shiny bracket where the sensing arm mounts to the differential housing:




Note: This works for me because I never run these vans empty. If however I unloaded the van(s), I would need to put the arm back to it's original position or risk stability problems (rear lock-up) in a panic stop situation.

[blackmetalspaceship]

Ok, so just did all four ball joints(thanx your article helped a ton!), inner and outer tie rods, and sway bar links. Than a new master cylinder to solve my recent squishy brakes.... still squishy. We follewed the lines to the lsp n bv valve, bled it there and no air. Front brakes no air when bled. Back brakes tons of air. No matter how much bleeding. No visible leaks in rear lines or drums. And, when you pump the pedal n release there are lots of air bubbles coming back up into the resevoir and you cant build up pressure. Soo, the thought is there is some tiny leak in the rear that slurps air but doesnt spray fluid, or the lsp bv is bad. My mechanic friends idea is to swap the lsp bv out with a manual proportioning block as we are unable to find a replacement lsp bv! Any ideas?

[timsrv]

when you pump the pedal n release there are lots of air bubbles coming back up into the resevoir and you cant build up pressure.............

Perhaps it's just the wording, but I wanted to make sure the bleeders were closed when releasing the pedal. Bleeders are only opened while pedal is being depressed, then closed before pedal is released. If you were releasing pedal with bleeders open, then go back and try to bleed again (with bleeders in the correct position).

As for your question regarding a possible leak, if there is a hole big enough to suck air, I can assure you it would leak brake fluid while pumping brakes............a lot (much higher pressure). The LSP & BV may have failed, but before condemning it, I would advise to disconnect the arm and put it at the highest possible spot, then try to bleed again. Be sure to hook it back up when you're done.

If you do all that & doesn't help, then maybe there's a bubble in the master cylinder. The master cylinder has 2 pressure chambers (one for the front & one for the back). It's possible there's an air bubble trapped in one of the chambers (this is the reason for bench bleeding...........did you do that?). Based on what you said it sounds like there might be air trapped in the chamber for the back. If air prevents pressure from building, then there is no pressure to push the air out. To correct that I would recommend removing the inlet line to the LSP & BV, then use a Mity-Vac to suck fluid. Have your friend watch the reservoir & add as required so it doesn't get sucked dry. After you get about 4 oz or so out of that brake line, hook it back up and try to bleed again. Good luck. Tim

[Jams]

Alright. This LSP BV thing is giving me a headache. I think i have a general idea on how it works. Im still wondering about a few things though. I noticed some fluid slowly leaking out of the boot on the main part of the valve, where the arm comes out. I will link a video somewhere to show yall whats going on. A few weeks ago i flushed the fluid by gravity bleeding it. Cracked all 4 bleeders (i was unaware of the 5th one at the time) and let the fluid drain out while topping the reservoir off, until clean fluid came out the bleeders. Brakes have been working pretty good since then. I have tested them by jamming on the brakes as if i were about to rear end someone (going 20-30 mph) and the back wheels didnt lock up, nor the fronts. Today i lifted my van by 7/8” (lets just call it 1”) by tightening the two torsion bars in the front. This seemed to lift the whole vehicle, front and rear. Maybe a little bit more in the front, i dont know. I have not driven it yet. I just learned from this useful site that adjusting the suspension can confuse the LSP BV and keep the brakes from working correctly. So ill probably just lower the van back to where it was until i figure the brakes out. Ok im kind of rambling, so here is my main question:

Is the LSP boot supposed to be filled with fluid? If you watch the video i am going to link, you will see that when i pull the boot back, fluid leaks out, and it looks kind of rusty. I wonder if i could zip tie the boot where it mates with the arm to help stop the slow leak. But im not even sure if that boot is supposed to have fluid in it. Perhaps it is an internal leak and fluid is wrongfully getting into the boot.

not sure if bleeding that 5th bleeder will help with anything, cause the brakes seem to be working ok, but ill give it shot at some point. Im going to do new pads, shoes and springs soon and keep working to get the brakes dialed in. BTW this is an ‘86 2wd auto van le.
-Jams

i uploaded a youtube video that kind of shows what im talking about:
https://youtu.be/8dzS8jVAVzA

[Diy2k]

Hey!

Have you tried making a lift bracket for the LSP? It’s a small bracket intended to lift the valve up and allow the brakes to work correctly. It made a difference on my van after I lifted everything (torsion and leaf lift)

[Jams]

I have not yet, but i think thats a great idea. I might try to lift my van another inch maybe and make an adjustable bracket with slots so i can try to find the right height for the arm. Thanks for the reply!
-jams

[alexpski]

Can the Load Sensing Valve be by-passed?

[timsrv]

Yes, but you would need some sort of standard proportioning valve to limit the rear brakes. Otherwise you would be most likely have rear wheel lock-up and loss of control in a hard braking scenario. Tim