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LSPV - What is it?

Discussion in '80-Series Tech' started by dekuiper, Apr 28, 2008.

  1. dekuiper

    dekuiper

    Messages:
    258
    I've been reading about brakes and master cylinder testing and I keep coming across this acronym. What is a LSPV?
  2. Grolar

    Grolar

    Messages:
    1,019
    Location:
    Bethesda, MD
    Load sensing proportioning valve. Google is your friend :)

    This is "a valve that attaches an arm to the rear axle, so that a heavily loaded bed compressed the suspension, which applies more braking power to the rear brakes".
  3. mainemike

    mainemike

    Messages:
    534
    Location:
    Central PA
    On your 80 it sits on the rear axle and can be adjusted to accommodate a lifted suspension. It's also a bleed location you should be aware of in addition to the 4 wheels.
  4. dekuiper

    dekuiper

    Messages:
    258
    Thanks all.
  5. beno

    beno 33030-60450 Moderator Supporting Vendor

    Messages:
    12,600
    Location:
    Covington, GA
    From Toyota Technical Information Services:

    "The LSPV is used on Toyota models such as Truck, Van and Station
    Wagons which may be used to carry a variety of loads. The heavier the
    load, the greater the portion of braking is required of the rear brakes.
    The LSPV allows higher pressure to the rear brakes to accomplish this.
    The LSPV is attached to the body or frame above the left rear control
    arm or axle housing. Load sensing is accomplished by suspending the
    sensing spring between the vehicle body and the rear axle housing. The
    load sensing spring movement caused by vehicle height changes due to
    load, is transmitted to the proportioning valve.

    As a vehicle is loaded, the leaf springs are compressed as the vehicle
    body lowers. The load sensing spring provides a variable force pushing
    the proportioning piston up as the vehicle is loaded. As the piston is
    lifted, a higher brake hydraulic pressure is required to force the piston
    down resulting in higher pressure at the rear wheels."

    There is more technical information if you are interested.

    -onur
  6. Squash

    Squash SILVER Star

    Messages:
    764
    Location:
    Sudbury, Ont. CA
    Think of LSPV as a primitive ABS system.

    As additional weight is added to your truck the tendency is for the rear end to lock up, a properly adjusted LSPV compenstes for load by reducing line pressure to the rear brakes.

    Have you ever tried a pull the handbrake stunt and locked your rear tires??? pretty hard to steer straight unless you release the brake.
  7. cruzer971

    cruzer971

    Messages:
    102
    Location:
    Prescott, AZ
    Are you sure about that?:hmm: Your post seems to be the opposite of the post right before it. And I thought his contained a direct quote from Toyota.
  8. Sarmajor

    Sarmajor

    Messages:
    209
    Location:
    Kaingaroa, Far North NZ,
    LSPV

    Unless my JDM is different you will not find the LSPV on the rear axle of an 80 series landcruiser. It is fited to the vehcle chassis cross member and there is a operating arm that is attahced to the rear axle of the vehicle.
    It is indeed a brake bleeding point and is often overlooked.

    Cheers
  9. Squash

    Squash SILVER Star

    Messages:
    764
    Location:
    Sudbury, Ont. CA
    Point taken about the FSM and Beno's post so maybe I require some schooling...Now

    "The most important point to understand about proportioning valves is when they work - only during panic braking. This means that a vehicle could potentially go its whole life without ever using its proportioning valve. Think about the little old lady that drives to and from church and the grocery store. It is possible that she will never have the need to make a panic stop and therefore never activate her proportioning valve. The other half of that equation is all those who drive near her will probably get a chance to see if their proportioning valves work! "

    Take your vehicle onto a dirt patch, drive 15-20mph and slam on your brakes. Get out and look at the four skid marks, your front tires should lock up first and most.
    If your rear wheels lock up greater than your fronts you will most likely head into the rubarb some day soon.

    Read this copy:
    PROPORTIONING VALVE & BRAKE BALANCE

    To reduce hydraulic pressure to the rear brakes so the rear brakes don't lock up when the brakes are applied, a "proportioning valve" is required. This valve helps compensate for the differences in weight distribution front-to-rear as well as the forward weight shift that occurs when the brakes are applied.

    What we're really talking about here is "brake balance" or "brake bias," which is the difference in the amount of hydraulic pressure channeled to the front and rear brakes. The front brakes on most rear-wheel-drive vehicles normally handle about 60-70 percent of the brake load. But on front-wheel-drive cars and minivans, as well as RWD and 4WD pickups and SUVs, the percentage handled by the front brakes can be as much as 90 percent of the load.

    [​IMG] Consequently, the front brakes need a higher percentage of the total hydraulic force that's applied to keep all four brakes properly balanced.

    If the front-to-rear brake force isn't balanced correctly by the proportioning valve, the rear brakes will receive too much brake force, causing them to lock up and skid when the brakes are applied. The other reason for using a proportioning valve to reduce hydraulic pressure to the rear brakes has to do with the design of the brakes themselves. When hydraulic pressure is applied to the wheel cylinder inside a drum brake, the shoes are pushed outward against the drum. When the shoes make contact, the rotation of the drum tries to drag them along. But since the shoes are anchored in place, the drum pulls the shoes up tighter only against itself. Because of this, drum brakes that are "self-energizing" require little additional pedal effort once the brakes are applied. Disc brakes, on the other hand, are not self-energizing. It takes increased pedal effort to squeeze the pads against the rotor.

    Some vehicles have load sensing proportioning valves that change rear brake metering to compensate for changes in vehicle loading and weight shifts that occur during braking. This type of proportioning valve has an adjustable linkage that connects to the rear suspension or axle. As the vehicle is loaded, ride height decreases and pressure to the rear brakes is increased. This type of proportioning valve can be found on many minivans, pickups and even some passenger cars.

    Load sensing proportioning valves usually are adjustable, and must be adjusted correctly if they are to properly balance the rear brakes to the vehicle's load. The valve linkage is adjusted with the suspension at its normal height (wheels on the ground) and the vehicle unloaded. The adjustment bracket or linkage is then adjusted according to the vehicle manufacturer's instructions, which typically involves adjusting the linkage to a certain position or height.

    Load-sensing proportioning valves are also calibrated to work with stock springs. Any suspension modifications that increase the load-carrying capability (installing helper springs, or overload or air-assist shocks, for example) may adversely affect the operation of this type of proportioning valve. Modifications that make the suspension stiffer reduce the amount of deflection in the suspension when the vehicle is loaded, which prevents the proportioning valve from increasing rear brake effort as much as it normally would. A defective proportioning valve, or one that is not properly adjusted, can also upset brake balance. If the rear brakes on a vehicle seem to be overly aggressive (too much pressure to the rear brakes), or the vehicle seems to take too long to stop (not enough pressure to the rear brakes), the problem may be a bad proportioning valve. Proportioning valves can be tested by installing a pair of hydraulic gauges (one on each side of the valve) to see if the valve reduces pressure as it should.

    On some late-model vehicles, the mechanical proportioning valve has been replaced by "electronic" brake proportioning through the ABS system. By sensing wheel speeds, the ABS system reduces pressure to the rear brakes as needed when the brakes are applied.
    Last edited: Apr 29, 2008
  10. mainemike

    mainemike

    Messages:
    534
    Location:
    Central PA
    yes, you are correct. I was just trying to point him in a general direction to eyeball the thing.

    mike
  11. Ebag333

    Ebag333

    Messages:
    8,163
    Location:
    Medford, OR
    Totally different purpose, I believe.

    What you're talking about is a mechanical type ABS system, where in certain situations the pressure to the rear brakes are reduced.

    The LSPV's purpose is the opposite, to increase the amount of braking by the rear brakes when there is more weight in the back. This is why when you have loaded your cargo area, or are towing a trailer, your truck still brakes evenly.

    I've towed with trucks that don't have anything similar, just straight lines to the rear brakes. The truck brakes totally different when pulling a trailer then when not. Braking with the 80 while towing feels nearly the same, due to the increased braking by the rear brakes.

    The attached picture (from the FSM) gives a simple idea of how it works. The more weight, the farther the piston gets pulled down, and the more fluid can squirt to the rear brakes, increasing the amount of braking power.

    Attached Files:

    • lspv.gif
      lspv.gif
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  12. Squash

    Squash SILVER Star

    Messages:
    764
    Location:
    Sudbury, Ont. CA
    Lexus/Toyota Tech

    Both senarios are correct.

    The LSVP does comensate for added weight and it will proportion brake force to the rear wheels during a panic stop.

    The following link provides additional info and tech some general and some specific to our units.

    http://www.autoshop101.com/forms/brake08.pdf
  13. rtarh2o

    rtarh2o

    Messages:
    936
    Location:
    NE Texas
    I always assumed the proportioning valve was always working. I think it is basically a pressure regulator since the rear brakes require less force than the front, without the valve you would lock-up the rear brakes with a normal load if they received the same pressure as the front brakes. The unique thing about the Land Cruiser valve is that it senses the load and adjusts the valve accordingly.
    Rusty
  14. rhyary

    rhyary SILVER Star

    Messages:
    1,402
    Location:
    Albany, NY
    If you have ABS, does the LSPV surve the same purpose as no ABS?

    I am about to install OME 2.5" and looking at the rusted LSPV I don't think it can be adjusted.
    Replacing sounds expensive.
  15. Squash

    Squash SILVER Star

    Messages:
    764
    Location:
    Sudbury, Ont. CA
    The LSVP is adjustable, they come with and without ABS equiped vehicles. The search feature and link above will offer some help as to lifts.

    Some folks use aftermarket units and adjust the to suit their needs, others have bypassed the unit completely, my concern here would be the potential to lock up the rear end on the highway.

    The valve is a non-serviable unit. Really soak the connections before trying to bleed.

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