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Brakes: Load Sensing Proportional Valve LSPV and LSPBV


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#1 twoblocked

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Posted 27 November 2011 - 05:12 PM

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....

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#2 bajadulce

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Posted 27 November 2011 - 05:31 PM

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.

#3 Maineah

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Posted 27 November 2011 - 05:57 PM

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.

#4 Derek up North

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Posted 27 November 2011 - 07:52 PM

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.

#5 twoblocked

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Posted 28 November 2011 - 06:19 AM

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

http://www.autoshop1...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.



#6 twoblocked

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Posted 28 November 2011 - 08:21 AM

>>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.
Maximum rear braking.small.JPG

"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.
Balanced frt-rear braking.small.JPG

"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.
Minimum rear braking.small.JPG

#7 bajadulce

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Posted 28 November 2011 - 02:52 PM

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.

#8 twoblocked

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Posted 28 November 2011 - 03:52 PM

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, 29 November 2011 - 01:53 PM.


#9 85mirage

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Posted 28 November 2011 - 03:58 PM

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!

#10 Derek up North

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Posted 28 November 2011 - 05:52 PM

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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