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Thread: Load Sensing Proportioning Valve and Bypass Valve ???

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    Load Sensing Proportioning Valve and Bypass Valve ???

    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?

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    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.

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

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    Re: Load Sensing Proportioning Valve and Bypass Valve ???

    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

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    Re: Load Sensing Proportioning Valve and Bypass Valve ???

    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!

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    Re: Load Sensing Proportioning Valve and Bypass Valve ???

    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.

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    Re: Load Sensing Proportioning Valve and Bypass Valve ???

    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?

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    Re: Load Sensing Proportioning Valve and Bypass Valve ???

    Quote Originally Posted by blackmetalspaceship View Post
    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

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    Re: Load Sensing Proportioning Valve and Bypass Valve ???

    Yeah, ive been closing the bleeder valve before releasing the pedal. And have bled thru two quarts of brake fluid. But i did fail to bench bleed the master, instead i pumped the pedal and cracked the lines one at a time coming from the master. It seems weird because the origanal problem came out of nowhere(no opening of brake lines or parts), and is still the same after installing the new master, so it seems like the problem is elswhere(and maybe the original master was ok). Ill try your suggestions, and maybe buy a mini vac, but do you have any idea where to get a new lsp bv, or something to bypass or replace it? Also, thanks sooo much, this website is SUCH a great tool and resource!

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    Re: Load Sensing Proportioning Valve and Bypass Valve ???

    I've never tried to purchase a new LSP & BV, but suspect Toyota would be your best bet. If they don't have them, then used may be your only option (I don't think the van specific ones were ever available through aftermarket). Sometimes parts like this can be disassembled, cleaned, then reassembled with new o-rings. I haven't needed to do that either, so I don't know if that's an option, just saying I'd try that before putting a used one on. Toyota's part number is 47910-28071 and the MSRP is about $230. The discount Toyota parts sites have them listed for ~$166. Tim

    PS: There's a couple reasonably priced aftermarket ones on eBay that look similar, but are not listed for the van. Perhaps there's a way to make one of these work or maybe you could make a good one between that and your old one??? Here's a link to one such listing: http://www.ebay.com/itm/92-97-TOYOTA...94fd82&vxp=mtr

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    Re: Load Sensing Proportioning Valve and Bypass Valve ???

    So if your car has ABS, this unit would no longer be needed correct?

    If (Generally,) a van had AntiLockBrackingSystem installed, then the need for this automatic valve would not be needed since full or equal pressure could be applied to all brakes equally, with the ABS being able to manage lock up?

    Or is Traction control needed as well to fully supplement this device?

    It seems to me that the real time feedback from the valve under normal conditions might be a good idea on this van, but I wonder how it would work on other platforms?

    Is this valve specific to this model? Or do other makes have them as well. This does not seem to be an option on any of my cars I have owned.


    Thanks,

    Mickey

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    Re: Load Sensing Proportioning Valve and Bypass Valve ???

    Proportioning valves have been around forever and are used on multiple makes & models. There are many different types. Most have a set limit (doesn't change). Some are adjustable (within a small range), and some universal ones (used on hot-rods & projects) have a huge range of adjustment. But none of these have the load sensing feature. These "non load sensing" type are typically set once then left alone as "close enough". Many models of Toyota use the load sensing type due to their ability to automatically change proportion with loading/unloading of the vehicle.

    As for your question on ABS I guess it would depend on the design of the system. ABS is for the all-out "OH SH%*!" moments and is rarely engaged. The LSP & BV is for keeping things in check during the normal stops (the other 99.9% of the time). Of course for non ABS vehicles the LSP & BV's role is more important during a panic stop. IMHO, for most vehicles the load sensing type proportion valve is overkill and can create some interesting problems when raising/lowering a vehicle and/or altering suspensions. Then there's the moving parts that make it more likely to fail (and that creates extra expense/hassle). When working correctly though, it's easy to understand how it can make the vehicle safer.........especially for utility vehicles like trucks & vans that are likely to be frequently loaded/unloaded. So it's a toss-up I guess & it's worth will depend on how you use the vehicle. I quit working for Toyota about the time the Previas came out (Previa's have these too) & not sure what they do on the newer rigs. I do believe though that Toyota may still be using these even on modern vehicles with full ABS. Tim

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    Re: Load Sensing Proportioning Valve and Bypass Valve ???

    Quote Originally Posted by timsrv View Post
    Sometimes parts like this can be disassembled, cleaned, then reassembled with new o-rings. I haven't needed to do that either, so I don't know if that's an option, just saying I'd try that before putting a used one on. Toyota's part number is 47910-28071 and the MSRP is about $230. The discount Toyota parts sites have them listed for ~$166. Tim

    PS: There's a couple reasonably priced aftermarket ones on eBay that look similar, but are not listed for the van. Perhaps there's a way to make one of these work or maybe you could make a good one between that and your old one??? Here's a link to one such listing: http://www.ebay.com/itm/92-97-TOYOTA...94fd82&vxp=mtr
    My 1993 Previa proportioning valve is leaking on top. Can it be disassembled and fixed? Can't find a used one.
    thanx

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    Re: Load Sensing Proportioning Valve and Bypass Valve ???

    Seems odd you can't find used. There are Previas all over the place here, but Michigan is a whole other market. If you can't find used then you'll probably need to go new. There's always a chance for repair, but you won't know until you take it apart to find the issue. I have never done this so I can't say if it's even possible. If you get it apart, at the very least it will probably need o-rings & I'm not aware of any rebuild kits or o-rings sold specifically for this purpose.

    If you find a used one, just remember that Previas with rear disc use a different valve than Previas with rear drum. New valves are available through Toyota for an MSRP of $270.54. Some of the discount Toyota parts sites will sell for ~$195.00 The part numbers are:

    Previa proportioning valve for Rear drum brakes = 47900-28201
    Previa proportioning valve for Rear disc brakes = 47900-28313

    I did a quick search on www.rockauto.com & eBay, but it doesn't appear that any aftermarket proportioning valves are available for the Previa. Good luck. Tim

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    Re: Load Sensing Proportioning Valve and Bypass Valve ???

    Thanks for all the good info. I tried Car-parts.com and the yards listing the part did not have it when contacted. Do you have any suggestions on a yard out your way that is reputable?

    Quote Originally Posted by timsrv View Post
    Seems odd you can't find used. There are Previas all over the place here, but Michigan is a whole other market. If you can't find used then you'll probably need to go new. There's always a chance for repair, but you won't know until you take it apart to find the issue. I have never done this so I can't say if it's even possible. If you get it apart, at the very least it will probably need o-rings & I'm not aware of any rebuild kits or o-rings sold specifically for this purpose.

    If you find a used one, just remember that Previas with rear disc use a different valve than Previas with rear drum. New valves are available through Toyota for an MSRP of $270.54. Some of the discount Toyota parts sites will sell for ~$195.00 The part numbers are:

    Previa proportioning valve for Rear drum brakes = 47900-28201
    Previa proportioning valve for Rear disc brakes = 47900-28313

    I did a quick search on www.rockauto.com & eBay, but it doesn't appear that any aftermarket proportioning valves are available for the Previa. Good luck. Tim

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    Re: Load Sensing Proportioning Valve and Bypass Valve ???

    Due to liability reasons many salvage yards will not sell used brake parts..........at least not knowingly. If you called and asked it's likely they just told you they didn't have it simply to avoid liability. This is the same reason most pick-n-pull type yards smash/destroy all the brake master cylinders before placing the vehicles in the yard. If I wanted a used proportioning valve I would go to a pick-n-pull, remove the valve myself, and if asked what it is I would say it's a clutch slave cylinder or some other non-brake related component. Good luck. Tim

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    Re: Load Sensing Proportioning Valve and Bypass Valve ???

    Crazy question...is it unsafe to drive if the leak in the proportioning valve is a "slow" one, I keep refilling the brake fluid to "max".

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    Re: Load Sensing Proportioning Valve and Bypass Valve ???

    The problem with leaks is they usually get worse. Due to liability reasons I or nobody else can tell you it's safe. Probably not a good idea to drive until the braking system is at 100%. Tim

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    Re: Load Sensing Proportioning Valve and Bypass Valve ???

    So after lifting my van with Mr Gasket Spacers (one on top of the coil and one underneath), I notice that my front brakes seem to be doing all the work all the time. I found this thread and my only question seems obvious but I can't grasp what I need to do. Do I need to add a bracket like Tim and raise my LPS up or do I need to drop it down? My guess is up, as the van is now higher than before. Not sure and having a hard time visualizing what is going on back there.

    Thanks,

    Justin

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    Re: Load Sensing Proportioning Valve and Bypass Valve ???

    Yes, if you lift the van you lift the arm. If you lower the van you lower the arm. Since the springs are still original, the same spring rates apply, so the adjustment would be the same as the height increase. Ideally you would want to measure the distance between the body and the rear axle before the mod, then measure it again after. If this dimension increased 1" then you would lift the arm 1" (where it mounts to the rear axle). Without prior measurements you would need to "guestimate" and adjust accordingly. Tim

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    Re: Load Sensing Proportioning Valve and Bypass Valve ???

    Thanks for the quick reply Tim as I would like to get this addressed as soon as possible (to be able to stop and whatnot ) I was picturing your van with weight in the back and figuring that since the weight lowers the van you countered it by raising the sensor. Therefore I was thinking that I needed to do the opposite. But I get it now. You just raised yours so that you will always have more braking in the rear than factory spec. I need to raise mine simply because my van is taller now. Thanks again. Need to bleed it too to see if any fluid is making its way back there. Am I going to have too much trouble finding the bleed valve?
    Last edited by JFratzke; 03-04-2014 at 08:48 PM.

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    Re: Load Sensing Proportioning Valve and Bypass Valve ???

    I raised my van with overload springs. So now when it's loaded heavy the chassis does not drop down as the factory intended. This is why I raised the mount on the rear axle. In my case, since I messed with spring rates my LSPV & BV will never work as intended. It would have made more sense to simply replace the LSPV & BV with an adjustable "non load sensing" type. Instead I raised the arm because it was the easiest/cheapest way to get my rear brakes back.

    On your van, if there's no air in the system then it's not necessary to bleed. You could put your rear axle on jack stands, then get you rear wheels spinning & verify you can stop them by applying the brakes. This isn't a substitute for a correct adjustment, but it will at least tell you if the rear brakes are working.

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