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Rear Wheels Locking Up

Discussion in 'Truck & Equipment Repair' started by jeh250, Nov 18, 2006.

  1. jeh250

    jeh250 Junior Member
    Messages: 9

    Anticipating winter, I just put the 8' Fisher plow back on my 89 F350. Problem is the rear wheels are locking up under relatively light braking. I understand the physics of the plow weight increasing the load on the front and possibly decreasing the load on the back, but this never happened before. Something has changed. The truck has the 460 engine, standard transmission, front disk, rear abs drum and gets very little use. Is there some sort of proportioning valve that may have gone bad? Thanks.
  2. SnoFarmer

    SnoFarmer PlowSite Fanatic
    from N,E. MN
    Messages: 9,883

    jeh250 "Is there some sort of proportioning valve that may have gone bad? Thanks."

    It very well could be... Take a look at the rear axle housing.
    There is a proportioning valve there.
    It could be a easy fix.
    Some times the rod that goes from the valve to the frame gets knocked off..
    pop it back on or you may need a new rod..

    just a thought, but at least it is a place to start.

    Was the truck sitting over the summer or any length of time?
    Of so it maybe rust build up on the drums..
    A few good stops should clean them up.
  3. jeh250

    jeh250 Junior Member
    Messages: 9

    Thanks for the feedback SnoFarmer. I'll look for that valve. It did sit for a couple of months waiting for a part, I'll clean up the drums too. BTW, there is no easy fix on this beast. I've thrown away all my wrenches. All you need to work on this one is a torch, grinder, chisel and a big hammer. New England salt. :)
  4. SnoFarmer

    SnoFarmer PlowSite Fanatic
    from N,E. MN
    Messages: 9,883

    yea.. I know about road salt...
    Duluth sits on-a-hill side, right on the shore of Lake superior.
    We get allot if freezing rain off of the lake.
    So with the hill the city uses a lot of salt.

    All you can do is make sure to wash your vehicles undercarriage, just after the storm and before the next cold wave hits.
    (never buy a used car from Duluth,MN. too much rust and no brakes):dizzy:

    Well, good luck on thoes brakes..
  5. Detroitdan

    Detroitdan PlowSite.com Addict
    Messages: 1,937

    On a couple different trucks over the years I have had rear axle seals go bad, they allow gear oil to leak onto the brake shoes, then when you first step on them they will be real grabby and lock up. Not sure why, seems like oil would do the opposite, but that's what happened to me a few times. So I'd pull the rear wheels and look, if it's real oily don't just change the shoes, change the seals too.
  6. jeh250

    jeh250 Junior Member
    Messages: 9

    Dan - You may have provided the answer. I changed the differential oil just before the problem started. Now that its full of fresh oil instead of running half full of black mud, it may be more prone to a seal leak. Thanks...
  7. mrbrickman

    mrbrickman Senior Member
    from montana
    Messages: 138

    if thats the case then id also get rid of the proportining valve too, better to give those brakes all the juice they can get...

    take it off and put a "t" in its place
  8. Rowski

    Rowski Senior Member
    Messages: 129

    Could have partailly stuck parking brake cable(s).

    If nothing is found wrong you could back off the rear brake adjusters a couple of clicks.

    Good luck.

  9. justme-

    justme- 2000 Club Member
    Messages: 2,138

    NEVER replace the valve with just a T. That's asking for an accident.
    The valve is there to regulate the amount of rear braking you get based on load. The truck is designed to carry a fair amout of weight- figure 3/4 ton- right? Do you drive with the full rated load in the back all the time? Then why on god's earth would you want the rear brakes to grab with the full pressure for carrying 3/4 ton all the time?! You'll lock them up every time you stop!

    Now, think about the problem for half a second- forget he added the plow, forget that the wieght has shifted. The brakes grab and the rear wheels lock up- why is adding more braking force to the rear going to fix that?

    Inspect the valve, but don't remove it.

    you mention ABS- there is a common prioblem with several ABS systems (Kelsey hayes in perticular) that the brake shoes being out of the range of the rear wheel ABS sensors confuse the system and it acts on it's own. My Ram has the issue from time to time. It'll lock up the rear for no reason when I stop sometimes, and sometimes a panic stop (like stopping on the highway at 70 empty) sometimes coupled with the proportion valve I will loose rear brakes all together. Try adjusting the rear shoes (back up @ 10mph and brake hard to a complete stop....a couple times) and see if it helps too.
  10. mrbrickman

    mrbrickman Senior Member
    from montana
    Messages: 138

    i know that i took that into account..and figure its a full time rust bucket plow buggy....i think he meant the front brakes were locking being that the backs were all oil soaked in which sense it need to be fixed right, new seals and new brakes or maybe soak them....

    when i lifted the k25 and added air shocks i had to remove the valve because it kept the rear end all the way up...which gave me no brakes in the rear, putting the t in hasnt caused me to lock the backs up any more than normal.....

    but i do see your point
  11. Rowski

    Rowski Senior Member
    Messages: 129

    Could you explain that one...

    Just very curious....
  12. porter5

    porter5 Junior Member
    Messages: 8

    1994 gmc just went to the shop casue the rear wheels kept locking up, would be backing up then boom it would just stop, then it was fine,happened for about 3 weeks then finally it dint move, had to get towed back to the shop, turned out a spring in the back left whell got caught and messed everything up, new spring everytihng works fine know....
  13. justme-

    justme- 2000 Club Member
    Messages: 2,138

    Mrbrickman- he said in the OP his rear were grabbing, someone suggested the leak on the rears.
    I've heard of many issues with rear lockups on unloaded trucks from removing the valve- there was a discussion on a diesel truck board I'm on last season about them and the concensus was to replace the auto valve with a manually adjustable one. hey sell them commonly to be installed in drag racers and race cars which can be mounted through the floor of the cab and let you adjust as conditions dictate. Newer trucks with softer front suspension seem to suffer more from the removal of the valve- probabily from the increased weight shift.

    Rowski: ABS brakes use a wheel rotation sensor to deturmine if the wheel is locked up. The system works by actuating a solonoid to pulse the brake system to the rear wheels in my case. The pulse is fractionally- in other words it does not move enough fluid to apply the brakes on it's own, it's really there simply to fight the pressure of my foot on the pedal and allow the rear wheels less brakes to turn a little again.
    Drum brakes are self adjusting in that when you reverse and them come to a complete stop (which we're all supposed to do before shifting into the opposite direction to prolong the life of our transmission and clutch) keeping them the correct distance from the drum as they wear. The mechinism that allows the self adjusting commonly rusts or binds on occasion, which we as driver's don;t notice because it will eventually break free in most cases and adjust as needed again.
    IF the adjustment binds backing up faster than normal (up to 10 to 15 mph) then stopping hard and completely usually breaks them free and asjusts the shoes,

    Now, if the adjuster is frozen for a while and the shoes wear out of proper clearence range the added foot pressure is usually imperceptible to us driving but the ABS solnoid moves so little fluid it can;t release the brake from our foot pressure. The ABS computer knows it's actuating the solonoid but the wheel rotation sensor doesn;t give the feedback the computer expects (the wheel is still not rotating) so the computer can open the solonoid in longer duration effectivly releasing the rear brakes longer, or depending on it's previous memory of the brakes condition it may clamp the solinoid down and lock up the rear wheels at the wrong time.

    Kelsey hayes systems are known for this in mid 1990's era vehicles. Particularly in the dodges. The same system was used in Ford trucks.
  14. Rowski

    Rowski Senior Member
    Messages: 129


    Nice explanation the "basic" ABS system. Originally I more interested as you mentioned earlier how ABS can work on its own by having drum brake out of adjustment.

    Some additional info about RWAL, Rear Wheel Anti Lock Brakes also known as single channel ABS.

    The ABS ECU has a pre determined (programed) rate of deceleration. It has no other way to "measure" how fast its decelerating in reference to actual. Speedo and ABS sensor are two separate sensors but are referenced by the same "drive", differential ring gear for ABS and speedo is from output shaft of transmission or transfer case. So once the ABS ECU see a deceleration rate faster than what is pre programed it will activate the ABS. It does not really care if its in lockup situation.

    There are three functions of most ABS systems; hold, release and apply. When the ECM sees a decel rate faster than preprogrammed it will release that pressure that is generated by master cylinder. It then allows the master cylinder pressure to "apply" the current master cylinder pressure again. If the deceleration is not faster than the preprogrammed value normal (non ABS) brake function will occur. If delec is still to fast, ABS ECU will hold that pressure and look at rate of deceleration, if to fast it will release pressure. The cycle starts over again. This cycle happens also very fast, not sure of the exact number but several times a second.

    A RWAL ABS system will never build more pressure in the brake lines than what is produced by the master cylinder. This is why during ABS stops it is best to let the ABS ECU do its job. Your stopping distance will be less than if you try to modulate the brake pedal while ABS is active.

    Out of adjusted rear brakes can and will cause problems with ABS performance. But it will not cause brakes to lock completely in ABS mode. Under long ABS activation stops rear brakes can become completely ineffective.

    RWAL is the simplest form of ABS. Three and four channel ABS systems get more complicated but the basic hold, release, apply cycle in the same.

    hope this helps...
  15. Detroitdan

    Detroitdan PlowSite.com Addict
    Messages: 1,937

    Think his 89 Ford has ABS in the rear?
  16. dilligafdog

    dilligafdog Member
    Messages: 53

    how often do you use this truck? some times with very little use the rear drums and every thing inside gets rusty from lack of use. some times it goes away with time and use, and some times you have to tear them down and clean and lube everything.
  17. oakvillerex

    oakvillerex Member
    Messages: 65

    If you have leaking axle seals you might want to check the spindles i have seen on a couple of plow/salt trucks the salt pitting the spindle so badly that the axle seal can no longer hold the gear oil in the rear end. It sounds like your rear brakes are just out of adjustment.
  18. justme-

    justme- 2000 Club Member
    Messages: 2,138

    I have routinly encountered rear brake failure from the ABS system in my Ram, and on occasion (usually shortly after such an incident) will have the rears lock up on very light pedal pressure. Issues with the system have been documented on several diesel and dodge diesel forums and sites. It was discussed on the boards and my mechanic confirmed he'd run into the issue and rear shoe adjustment was the major culprit.

    I have had half a mind to pull the RWAL fuse from the day I bought the truck- they have never actuated properly functioning in a panic stop, but I'm aware some vehicles have major issues with inoperative ABS systems, so I was never sure enough to do so.

    Good explanation also- I'm going to have to delv into the Kelsey Hayes manual some more....
  19. Detroitdan

    Detroitdan PlowSite.com Addict
    Messages: 1,937

    I'm still voting for gear oil on the shoes. Hurry up and rip those rear wheels off -enquiring minds want to know!
  20. jeh250

    jeh250 Junior Member
    Messages: 9

    I will, I will. It's Thanksgiving and raining cats and dogs here in Connecticut. Perhaps tomorrow. :salute: