twoblocked

Brakes: Load Sensing Proportional Valve LSPV and LSPBV

67 posts in this topic

I've been following a lot of threads on this subject. There is much misinformation out there. Maybe we could start with the following PDF attachments: the Toyota FSM and the Toyota Lexus Training Manual. (it refers to our Trucks also)

Granted, this is for the technically minded....

Toyota FSM-LSP&BV.pdf

Toyota Training Manual-LSPV.pdf

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thnx for uploading. Ya, quite techinical, but good info. Look forward to reading more input from members on this subject especially those of us with airbags.

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The MH is all ways at max load plus some no matter what. I'm going to say that the chance of locking the rear wheels is all most nill even with full pressure so it's best to have all the breaking you can get back there. Unlike a car with the weight transfer forward with the rear lifting the MH will not lift at the back because of the weight so there is a lot more breaking going on in the back of a loaded truck then in a normal car or and unloaded pickup. Shortly after I got mine I by passed the valve I will say that the results were not dramatic it still took 90% more then a dime to stop it.

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Did you manage to find the Training Manual online? Thanks for posting the section. I'd like to read other sections, if they're available somewhere.

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Yes, the complete Training Manual is available online at Kevin Sullivan's Autoshop 101 web site.

http://www.autoshop101.com/autoshop15.html

Fascinating bedtime reading......

Did you manage to find the Training Manual online? Thanks for posting the section. I'd like to read other sections, if they're available somewhere.

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>>The MH is all ways at max load....

Agreed. My Itasca has 2,000# on the front and 4,000# on the rear. Front disc brakes, rear drums.

>>I'm going to say that the chance of locking the rear wheels is all most nill

Again agreed. (icy conditions exempted)

>> ... the MH will not lift at the back because of the weight so there is a lot more breaking going on in the back of a loaded truck....

I'm having difficulty getting my head around this statement. Any vehicle in heavy braking conditions moves weight forward due to inertia. Just because our MH have more weight in the back does not mean this is where the majority of the braking power should be.

I believe DRAGGING (rear wheel bias) the duals is a less efficient way to stop than the judicious use of HIGHER braking pressure in front (front wheel bias) and LOWER braking pressure in the rear. A properly adjusted LSPV assures this.

This is what adjusting the shackle on the LSVP spring does for my braking; Pics show the adjustment changes.

"Maximum Rear Braking" adjustment feels like most of the braking is being done by the rear wheels. There is no diving of the front end under any conditions.

post-4694-0-80137000-1322496959_thumb.jp

"Balanced Frt-Rear" adjustment was the original position when I got the MH. The front of the MH dives under hard braking. It feel like a normal vehicle, both front and rear are working.

post-4694-0-91834500-1322496975_thumb.jp

"Minimum Rear Braking" adjustment causes the front to severely dive under hard braking. I don't think the rear drums are doing much at all.

post-4694-0-27690800-1322497000_thumb.jp

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I believe DRAGGING (rear wheel bias) the duals is a less efficient way to stop than the judicious use of HIGHER braking pressure in front (front wheel bias) and LOWER braking pressure in the rear. A properly adjusted LSPV assures this.
So the best braking would be a balanced Frnt-Rear adjustment such as your second picture or a more maxed rear? Maineah of course bypassing his entirely.

WME brought up a good point in that air bags negate the intended purpose of this valve (truck sits at designated level based on air pressure in bags not spring plate to frame displacement). In my particular scenario, the camper comes off the rig and thus the truck has 2 distinct loads (varying loads is exactly what this valve is designed for). However with airbags, it was suggested the setup be modified so that it has 2 quick settings; one for load (either camper on or w/work payload) + one for empty truck (camper off/no payload) which seems like a great idea. If not, I guess the easiest solution would be to find a middle ground for this scenario? A bypassed or Max setting valve would prob be Ok for the full payload and most RV's, it would probably not be so great for an empty truck w/ no camper.

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I don't see any problem with increasing rear wheel bias in our rigs due to the constant loading.

The issue is when the LSPV is eliminated entirely from the braking circuit. Disc brakes by their design require higher fluid pressure to operate as compared to a drum brake. The reason being the pads in the calipers clamp the rotor, more stopping power requires higher and higher pressures. A drum brake is "self energising". This means that when the brake shoe is pushed into the drum, the rotational force actually pulls the shoes more tightly against the drum. I've read references that stated a caliper/disc brake requires two to three times the fluid pressure of a comparable drum.

So what this means is by eliminating the LSPV you have equalized the front/rear braking pressures in the system and the rear drums are exerting a larger braking force than the front discs. I personally feel you have just negated a substantial portion of your braking power by doing this. I have nothing but empirical evidence to back me up here (my own) so if I am off base, I'm listening.

WME's point is spot on. When you change the pressure in your air springs, the LSPV is also re-adjusted automagically.

In your case of light and heavy loads I think the coolest solution would be an adjustable proportional valve to change the rear brake bias when needed. The gear heads do it all the time. Most eliminate the LSVP (be sure to cap the front brake sensing circuit also, it's the second line coming from forward into the LSPV) and put a knob operated adjustable proportional valve up under the hood, near the Master Cylinder. You could fine tune for both scenarios.

So the best braking would be a balanced Frnt-Rear adjustment such as your second picture or a more maxed rear? Maineah of course bypassing his entirely.

WME brought up a good point in that air bags negate the intended purpose of this valve (truck sits at designated level based on air pressure in bags not spring plate to frame displacement). In my particular scenario, the camper comes off the rig and thus the truck has 2 distinct loads (varying loads is exactly what this valve is designed for). However with airbags, it was suggested the setup be modified so that it has 2 quick settings; one for load (either camper on or w/work payload) + one for empty truck (camper off/no payload) which seems like a great idea. If not, I guess the easiest solution would be to find a middle ground for this scenario? A bypassed or Max setting valve would prob be Ok for the full payload and most RV's, it would probably not be so great for an empty truck w/ no camper.

Edited by twoblocked

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I love the picts. I had my valve adjusted to more in the rear awhile ago. I think it needs it again,can this be?. But i never knew what exactly was being adjusted. Picts make easy sense. I think my MH was adjusted like the first pic. Made MH stop a lot quicker!

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Thanks. Always being nosy, I clicked around and came across the following, listed as being for instructors! :ThumbUp:

http://www.autoshop1...autoshop31.html

I'll let you know if I find anything else. :rolleyes:

Yes, the complete Training Manual is available online at Kevin Sullivan's Autoshop 101 web site.

http://www.autoshop1...autoshop15.html

Fascinating bedtime reading......

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Yes, this can be.

As one increases rear braking pressures, there is additional wear on the rear brake shoes. This causes them to wear prematurely. You could be experiencing rear brake shoe wear. Or simply out of adjustment brake shoes.

Anecdotal evidence from one gear head poster on another blog was that after he increased rear braking bias he wore out rear brake shoes at a rate of 2:1 (two pairs of rear shoes for every one set of front pads). Before he increased rear bias he was wearing out front disc pads at the rate of 2:1. (Two pair of front pads for every pair of rear shoes)

Makes sense. It's the price of admission for jacking up rear bias.

I think it needs it again,can this be?

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I don't have a problem with brake pads or shoes wearing out, as long as they're providing useful stopping effort in return. I'd rather pay for pads/shoes than bodywork (or funeral expenses)!

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Yes, this can be.

As one increases rear braking pressures, there is additional wear on the rear brake shoes. This causes them to wear prematurely. You could be experiencing rear brake shoe wear. Or simply out of adjustment brake shoes.

thanks i'll check rear pads, As the rear pads wear down do the nuts on the valve move down causing more front pressure? and need to readjust vavle after new pads?

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Brake shoe wear (or installing new shoes) will not affect the valve. Changing your static ride height will require the valve to be adjusted.

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... However with airbags, it was suggested the setup be modified so that it has 2 quick settings; one for load (either camper on or w/work payload) + one for empty truck (camper off/no payload) which seems like a great idea. If not, I guess the easiest solution would be to find a middle ground for this scenario? A bypassed or Max setting valve would prob be Ok for the full payload and most RV's, it would probably not be so great for an empty truck w/ no camper.

I think that if I was in your position, I'd load the camper on (full of supplies as if you were leaveing for Baja) and adjust the rod so that the valve is fully open. Then remove the camper and check that the valve position has now changed. Then take it out on a quiet road and stomp on the brakes and see if the rear brakes lock up before the front. This would not be good. A sure recipe for the truck to swap ends. But the bottom line is that you're going to have to accept that the rear is going to have to ride at least a bit lower with the camper on, even if your air springs are able to lift it back to 'Empty' height. The valve isn't particularly 'intelligent'. It has only 1 source of information. Your new air springs can very easily fool it.

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Oh boy !! Reading brake08.pdf made me realize for sure what I've been thinking about the rear brakes. I have watched the body rise/fall as I varied the pressure in the air bags. As the axel does not move the valve adjustment must surely change. Not only do we have a whole lotta weight on these rigs but we also have much much more overhang beyond the rear axel than Toyota designed for. I don't think I'm getting much from the rears now, braking while backing downhill is a joke , the fronts lock easily and just slide. I have learned to apply the rears with the hand brake to get it to stop, now I see why. so I'm gonna set that thing to perment max braking and see how well that works.

vanman

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... braking while backing downhill is a joke , the fronts lock easily and just slide.

Particularly 'fun' if you happen to have a trailer hooked up. Speaking from personal experience, though not with a Toyota motorhome involved. Somehow I managed to not jack-knife! Some praying was involved!

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Just finished doing the adjustment on mine. Unfortunately mine does not have screws to adjust top and bottom. On mine I had to remove the bar and unscrew the whole top unit and them reinstall the bar. haven't check the brakes yet but I'll check back and tell you the difference. Was down pretty low so the front brakes have been taking the most wear. Back should be like new

LS

>>The MH is all ways at max load....

Agreed. My Itasca has 2,000# on the front and 4,000# on the rear. Front disc brakes, rear drums.

>>I'm going to say that the chance of locking the rear wheels is all most nill

Again agreed. (icy conditions exempted)

>> ... the MH will not lift at the back because of the weight so there is a lot more breaking going on in the back of a loaded truck....

I'm having difficulty getting my head around this statement. Any vehicle in heavy braking conditions moves weight forward due to inertia. Just because our MH have more weight in the back does not mean this is where the majority of the braking power should be.

I believe DRAGGING (rear wheel bias) the duals is a less efficient way to stop than the judicious use of HIGHER braking pressure in front (front wheel bias) and LOWER braking pressure in the rear. A properly adjusted LSPV assures this.

This is what adjusting the shackle on the LSVP spring does for my braking; Pics show the adjustment changes.

"Maximum Rear Braking" adjustment feels like most of the braking is being done by the rear wheels. There is no diving of the front end under any conditions.

post-4694-0-80137000-1322496959_thumb.jp

"Balanced Frt-Rear" adjustment was the original position when I got the MH. The front of the MH dives under hard braking. It feel like a normal vehicle, both front and rear are working.

post-4694-0-91834500-1322496975_thumb.jp

"Minimum Rear Braking" adjustment causes the front to severely dive under hard braking. I don't think the rear drums are doing much at all.

post-4694-0-27690800-1322497000_thumb.jp

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Just finished doing the adjustment on mine. Unfortunately mine does not have screws to adjust top and bottom. On mine I had to remove the bar and unscrew the whole top unit and them reinstall the bar. haven't check the brakes yet but I'll check back and tell you the difference. Was down pretty low so the front brakes have been taking the most wear. Back should be like new

LS

So here is how my adjustment worked out. Had no effect what so ever. Just got back from a long trip and on some pretty rough roads. I noticed absolutely no difference in the braking. Whats up?

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Just remove the arm from the adjustment bolt and tie the arm to the frame. If still no change, then you may have a bad regulator.

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I was just reading over my 1990 Toyota Factory Truck repair manual about Load Sensing Proportionging level adjustment

and it states that in additon to adjusting the height of the lever that the valve body itself can be raised or lowered and shows

a diagram of the valve body and appear to be a bolt or two in the middle of the body that when loosened allows the valve

body to move up or down. I am going to give this a try in the next day or two.

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I was just reading over my 1990 Toyota Factory Truck repair manual about Load Sensing Proportionging level adjustment

and it states that in additon to adjusting the height of the lever that the valve body itself can be raised or lowered and shows

a diagram of the valve body and appear to be a bolt or two in the middle of the body that when loosened allows the valve

body to move up or down. I am going to give this a try in the next day or two.

Those factory service manuals are just too large for my old computer to handle.I would be eternally grateful if you could post some pics from it so I could see if I can adjust mine some more. Just the proportioning valve adjustment didn't really help on mine

Linda S

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And that is exactly what I would like to do. I would rather not take the thing apart if I can. adjust it more some other way

LS

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Without benefit of seeing the FSM section in question I'm only 'guessing' (which I don't like doing). But it sounds as though doing this adjustment won't accomplish anything IF you've disconnected the linkage and secured it in the fully 'Up' position. It sounds like the additional mounting holes are there in case the rod/linkage won't adjust enough while still connected.

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Ok here is the deal on rear truck brakes. Some large trucks (dump trucks etc.) have no front wheel breaks when you place most of the weight in the rear there is not nearly as much weight shift to the front it does not lift. A Toyota MH is all ways weighted to the rear it needs all the rear brakes you can give it under any circumstance . Backing up with drum breaks will be poor breaking because of the design of drum breaks they are what is called self energizing meaning the forward motion help apply them this does not happen in reverse. If you decide to adjust the proportioning valve adjust it for absolute max load because that is what it is.

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