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Brake Proportioning Valve

Discussion in '80-Series Tech' started by concretejungle, Oct 31, 2003.

  1. concretejungle

    concretejungle SILVER Star

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    I remember reading that you need to re-adjust the rear brake proportioning vavle after a lift. From what i understand, it regulates the amount of braking power applied to the rear based on the amount of weight in the truck. I just got the 2.5 lift on (awesome by the way :D) and i'm wondering if this is something i really need to do?
     
  2. Beowulf

    Beowulf

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    CJ,
    Yes, you need to adjust the LSPV. Lower the bracket .25"

    That doesn't sound like much but it is important and will make a difference. When you lower it, make sure you have plenty of light back there so you can clearly see what you are doing. I thought I made a reference mark on the frame and the bracket so I could use that to see how much it was being lowered. The mark was on 2 parts of the bracket so I lost my reference and had to guess. I lowered it too much and went through a brand new set of rear pads in about 10k miles. Lowering the LSPV bracket is very easy to do.
    -B-
     
  3. Riley

    Riley

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    I'm getting ready to put on the OME 851/860 lift. I assume that I need to adjust the LSVP as well?

    The same .25".

    Can someone explain how the LSVP works and why I'm doing this? ???

    Just that I like to understand what's going on before I mess with it.

    I'll do my bedtime reading in the FSM tonight so maybe that will help.

    Thanks

    Riley
     
  4. Beowulf

    Beowulf

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    The FSM will explain the LSPV.

    Basically, it senses when there is more load in the cargo area and increases the amount of load the rear brakes have in the system. When you raise the body with the 50mm lift (yes, you need to with the 851/860's) the LSPV thinks there is no load in the cargo area so the rear brakes don't do their share of the stopping.

    The .25" was determined by others long, long ago. It is accurate. Basically, there is a 16:1 ratio of movement. The long arm cannot be easily adjusted to lower it 50mm (2.5") so by lowering the bracket, the LSPV is adjusted close to the correct percentage.

    The FSM says to use a complex method requiring special brake equipment that nobody has.

    -B-
     
  5. Rich

    Rich

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    If you want to follow the factory procedure here is what you need to buy. You need two. They cost $39 each. They fasten directly into the brake calipers replacing the bleeders. You also need two buddies, one to watch the front gauge, and one to apply the brakes while you monitor the rear gauge and adjust the porportioning valve. I purchased my gauges from Classic Industries.

    Rich
     
  6. Riley

    Riley

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    [quote author=Beowulf link=board=2;threadid=6969;start=msg58354#msg58354 date=1067837706]
    The .25" was determined by others long, long ago.
    [/quote]

    Kind of reminds me of "Planet of the apes".

    But I shall follow as I don't think I've got 2 buddies that I trust under my truck with gauges. (1 buddy yes).

    ;)

    R
     
  7. landtank

    landtank SILVER Star

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    The factory propotioning valve is tuned to the factory spring rate. A certain amount of weight produces and expected amount of compression. Once you respring your truck with something heavier that relationship is gone. I wouldn't bother with the factory adjustment as it really won't be any more acurate than what -B- has suggested.
     
  8. concretejungle

    concretejungle SILVER Star

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    Adjusted the brake proportionate vavle yesterday. Very simple to do. I believe i can tell a difference. Could be mental, but i guess i'll know if the front pads wear in 3,000 miles :D.
     
  9. Rich

    Rich

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    In practical terms the objective of the proportioning valve is to prevent the rear brakes from prematurely locking up and causing a skid with resultant lose of directional control while at the same time increasing the rear wheel braking (relative to front wheels) when there is a heavier load in the rear and therefore more rear wheel traction and therefore less likelihood of rear wheel skidding.

    The gauges are just a tool that let you directly measuring how the proportioning valve is actually proportioning the hydraulic pressure.

    It is true that the rate that the valve increases rear hydraulic pressure relative to weight is affected by the spring rate. But there is nothing you can do to tune the valve rate unless you modify the overall length of the long arm and change where it mounts to the body.

    The factory procedure results in a specific proportioning being set when the vehicle is, relatively speaking, lightly loaded. This is the situation where there is the greatest risk of skidding resulting from too much rear breaking.

    Even with stiffer aftermarket springs, if the factory engineers got it right the first time, I believe that the factory procedure is still appropriate.

    I understand that many would not choose to spend $80 bucks on tools that may only be used one or two times. For me, it allowed me to get the brakes adjusted according to what the Toyota engineers think is appropriate. When compared to the cost of the lift, sliders, and rear bumper, $80 bucks in itself wasn't a big factor.

    But if you really want to know, I got the gauges because I am fussy. Think of the gauges as being torque wrenches for your brake system. They just make the job easier to get right.

    Rich
     
  10. IdahoDoug

    IdahoDoug

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    It would be interesting to use that gage set to see what the proportional split is with the standard .25 inch movement, vs the factory split. You know? To see how close that bracket movement comes to what adjusting the valve does. Unless the valve arm adjustment simply doesn't have the range to accomplish the comparo?

    Doug

    Doug
     
  11. Rich

    Rich

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    Doug,

    I first adjusted the adjustmet and the axle end, and the total range of that adjustment did not come close to compensating for the lift. So I then adjusted the arm mounting bracket on the frame. At the time, I did not measure how much I adjusted either. One thing I did notice is that the entire valve mechanism is "sticky" when the truck is not in motion (like parked in my driveway). After making changes to the bracket position, I needed my buddy to jump up and down on the bumper, and then get off while we applied the brakes and checked the gauges. I am sure in normal use the movement of the suspension operates the valve just fine.

    Rich
     
    Last edited: Jun 18, 2005
  12. landtank

    landtank SILVER Star

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    Being the Maverick I am :p I went a totally different route. I loaded my truck with the usual junk I carry and measured the diffence between the axle and frame. Then after the springs where installed I remeasured and then bent the rod to compensate. That way I was assured that the braking force hadn't changed for my usual load. Worked for me.
     
  13. concretejungle

    concretejungle SILVER Star

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    landtank, i read in the 80s cool technical section about adjustment on the BPV. They said under no circumstances should you bend the rod.

    To all you like minded folks out there, no smart xxx remarks about bending your rods ;). :flipoff2:
     
  14. landtank

    landtank SILVER Star

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    It's important to realize how the arm interacts with the valve. I bent the arm beyond that point of where they interact. I traced the arm out and then pivoted the arm to get the proper drop, marked that spot and then bent it so the end hit the target but did not disturb that area at the valve.

    As far as 80scool goes there is alot of good info but not everything is GOLD as I see it. As an example, one write up remarks about the faulty pinion bearings in the front diffs. Another write up talks about greasing the front drive shafts and getting vibrations. You could look at those seperately and see 2 different problems. I prefer to see them as one. Getting front end vibrations after greasing indicates a pinion problem and removing the grease is only treating the symptom and not the problem. I did have my truck regeared and the pinion bearing was needing replacement long before I had the work done. I now grease the front shaft "improperly" and have no vibrations.

    I'm not saying I'm right or my methods are superior, just that this is my way of thinking and if someone can show me that I'm missing something than I'm all the happier. 30k til my next beerfield repack :D :D :D
     
  15. Junk

    Junk

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    If you go with J springs or more, spend the extra few mins and fab up a new bracket for the LSPV. You'll need it as it maxes out.
     
  16. Riley

    Riley

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    [quote author=Beowulf link=board=2;threadid=6969;start=msg57495#msg57495 date=1067632775]
    CJ,
    Yes, you need to adjust the LSPV. Lower the bracket .25"

    That doesn't sound like much but it is important and will make a difference. When you lower it, make sure you have plenty of light back there so you can clearly see what you are doing. I thought I made a reference mark on the frame and the bracket so I could use that to see how much it was being lowered. The mark was on 2 parts of the bracket so I lost my reference and had to guess. I lowered it too much and went through a brand new set of rear pads in about 10k miles. Lowering the LSPV bracket is very easy to do.
    -B-

    [/quote]

    I'm getting around to this today. Just to confirm - after marking the current position, I'm going to loosen the nuts on the set plate bolts that hold the valve to the valve bracket (using terms from the FSM, BR-30)?

    Looking at the truck, I can't see how the bracket mounts to the frame in a fashion that would allow adjustment.

    This sounds like :doh: question and I'm taking the terms a little too literal.

    I'll go look at this again and reserve the right to edit this post if I'm just being dense.

    R

    Edit #1 - After having another look, I'm actually adjusting the valve with respect to the bracket. Moving the valve down by .25" on the bracket. I'll go for a test drive to check it out.

    Edit #2 - On my test drive I was doing about 50 mph and hit the brakes hard. The ABS didn't kick in and I left a little rubber on the road, I think from the rear tires. My guess was that I went too far moving the bracket. It was a healthy .25".

    Is the ABS system only on the front wheels or rear as well?

    I backed off the .25" and another test drive. No rubber this time. Seems ok and better than before I started. Next on the list is new brake fluid (synth) and new pads.

    Man it's hard to get that ABS to fire on a dry cold road, perhaps that's normal for the 80.
     
  17. Darwood

    Darwood

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    So by lowering the LSPV we are to lower the valve that is attached to the bracket that is then attached to the frame? I tried this and was only able to lower it about .25" due to binding of the brake lines and just general unwillingness to be lowered more.

    Also what sort of bracket would one fab up if they had J springs?
     
  18. clownmidget

    clownmidget

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  19. Darwood

    Darwood

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    What Landtank did is somewhat similar to the bracket I made to help with the LSPV. I probably should have mentioned that. However instead of adding to the adjustment bolt I raised the bracket that the bolt is connected to. I did find that I couldn't raise this bolt very much since raising it too much caused the LSPV arm to contact the upper control arm.

    When I get my cruiser back I'll post pics.
     
  20. clownmidget

    clownmidget

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    Derek, I'll be interested in seeing what you did. I made mounting plates to deal with this on my 4Runner and Tacoma but I was amazed at how elegantly simple landtank's approach really was. I was given a scrap piece of gas pipe about 18" long and the the tap was a whopping $3.85. The bolt was another $0.60. It took me about 10 minutes to cut the pipe to length, tap it, and mount it up. Like I said, I am amazed and this ranks right up there with the best cheap tricks. It's got a total of about 2" of very fine adjustment too so it should be possible to "tune" the LSPV to suit your needs.

    Mike