Adjusting Load Sensing Valve on M1078

I'm trying to troubleshoot a possible malfunction with my "Park" brake. It has partially activated several times when hauling a load. Oddly it happens when the air dryer purges. It hasn't happened when the truck is empty so I'm suspicious it has some relationship to the Load Sensing valve. My air pressure for both primary and secondary tanks are at 120psi when the dryer purges. The rear end Growls and vibrates and creates a drag as soon as the purge happens and forces a complete stop of the truck. Driving returns to normal until the next purge.
I just can't find any troubleshooting info that discusses the Load Sensing valve?
My guess is I may have a check valve failing allowing a brief loss of pressure signaling my Park (Spring) brake to apply.
Would really appreciate some input please!!!

Wow add this to the list of bizarre FMTV air brake problems. Have not seen this, my only suggestion is to go back to the usual culprit of spring brake application while driving, issues with the vent in the front gladhand covers.

talk to suprman / Will. Believe he has had something like this issue happen to him as well.

Your not kidding! This could easily turn into a catastrophic event. I adjusted the Load Sensing valve last night and tried to reproduce the event on a gravel road but couldn't get it to act up. I did however get an idea of how sensitive the rear brakes get when loosening the cable on the valve lever. I could get the primaries to lock with ease. I then backed it off to the other extreme until the front brakes were locking. There's got to be a better way to to find the proper "middle ground "for the adjustment! It is kinda important !! Thanks for the replies...

Moderator can delete this post.... had wrong valve in mind

FYI... the valve is mounted under the bed and has a cable which attaches to a spring mounted on the axle. When the bed is carrying a load heavy enough to compress the leaf springs, the cable will develop slack allowing the lever on the valve (proportioning valve) to rise. The valve then increases the air pressure dedicated to the rear brakes. This is my understanding anyway....

Wow glad I say this thread in case I have trouble. All I could find is the TM hope it is an easy fix.

There seems to be a correct way to set it up so if you have not read the whole TM. I scanned the first page to give you the TM number and what page it starts on. Maybe it's like the old brake system on a car that had the ball in the line at the master cylinder and you had to bleed the system one side or another to get the ball centered so the light would go out. Maybe if it is set up the correct way the system will work correct. There were a lot of steps so read the whole section. Hope it helps.

Nice!! Have this TM but didn't know where this was. Much thanks! I'll dig right in...

Don't know but seems like every time the passenger side glad hand is talked about when there is a problem to. The end hand has to have the vent and the one way check valve has to be working so as not to push air back into the braking system.
Yea Will seems to be pretty hot on the FMTV stuff. You out there Will.

Ya, I keep studying the schematics and that emergency gladhand is "Pre-wet tank" so it could definitely be affected by a change in pressure. What is also interesting is there is a two way check valve between a low pressure switch and the air dryer within that same airline. Could be if too much air is purged that this low pressure switch gets tripped.
When I got this truck I had to do a complete brake job on the drivers rear. It had seized up and disentigrated the brakes so badly that part of the liners welded to the drum. Other brake liner debris spewed onto the truck axle,frame and shock. The interesting thing is the shock was abviously compressed at the time this event took place because it was perfectly clean where the top shock cover would have been if the truck was heavily loaded. The bottom brake shoe was installed the wrong way so at the time of the brake job I blamed the knucklehead who did the brakes for the failure.
Now I'm not so sure! I think what I'm dealing with now could be the the underlying problem. Wouldn't we all love to know more about our trucks history?

untill one gets this sorted out.... could not the load sensing valve just be bypassed?

Yep! I might just do that if it happens again. At the present I have the valve setting backed off to the least aggressive setting. Still not sure if I'm on the right trac?

coachgeo said:

untill one gets this sorted out.... could not the load sensing valve just be bypassed?

Click to expand...

I don' know and am reading as much as I can. As long as bypassing or trying to get rid of it did not cause the wheels to lock up even with a light touch of the brake peddle when empty?

I'm with you ....the engineers didn't put on there just for looks. Plan is to trial it this weekend with a load of gravel. The adjustment I made is pretty significant and if it does act up it should be a much milder event. Still a little scared! Anybody brave enough to ride shotgun?

as I read about these... it appears there is several functions.... one it adjust for load placed in bed (obviously) but not only that but some of these valves also are for auto adjusting of brakes base on " lifting of rear" which lessens the effect of rear brakes due to less tire pressure on ground..... such as when one brakes hard you get a lift at the rear ..... appears this lift is compensated for by this valve as well cause it senses the lift.

Now with that in mind......... if your shocks SUCK or other suspension component is worn??? would this not also increase the rear lift in braking and cause this valve to activate..... maybe more than it should?? Like if front shocks suck..... the nose will dive more and pull the rear up??

Glad to be able to bounce this back & forth!! The cable that mounts to the axle is on a spring. So what happens when there would be a "hop" or rise in the rear suspension is this mounting spring stretches which allows the cable to move upward in unison with the valve so no change in valve action occurs. Simple but clever design! I'd really be curious to see the position of anyone else's valve/cable/spring setting. Anyway, the craziness here is my spring brakes applied on flat smooth ground when I was not applying the brakes.
I checked out the air brake troubleshooting link you posted. Really, really appreciate your help!!

When you said "front brakes were locking" did you mean literally the front brakes on the truck (and not the secondary on the rear brakes) ?

Consider this; The front brakes do not have spring brakes to lock up. If the front brakes are locking then it's from application of air pressure, not air loss, that locks them. The rear brakes could also lock up from unwanted application of air. You said the front and rear brake tanks had plenty of air, and it takes a loss of air to get spring brake lockup, and only in the back. I do not think it's your spring brakes locking up.

Now if you had some air somehow going into the brake lines to both rear and front, apparently caused, or effected, by the load sensing switch, then it's not a spring brake lockup but unwanted application of the brakes.

So how could that be happening? It's not caused by treadle valve 'activation' because you are not applying the brakes, right? [edit; see below*] The treadle valve is two separate circuits running through the same foot-operated valve so unless two circuits were failing the same way (unlikely) the failure is happening past the treadle valve(s), to something common to rear and front.

Every load-sensitive brake system I am familiar with only effects the rear brakes. The need is simply because you want the rear brakes to carry more of the load of stopping the truck as the load increases, otherwise the rear brakes do lock up with light load (opposite what you have happening). But the load sensing system must be causing some common element to pressurize both systems.

* [but if you meant that the front brakes started locking up when you tried to brake, then that means you adjusted the rear brakes to minimum pressure, they are not doing their share, therefore the front brakes skid and lock up because they are not getting help from the rear brakes. That would be normal, and you are looking at only a rear brake activation problem.]

I can't find the air system diagram in my manuals to look at, but if someone has the manual number and page number handy, maybe we can guess further.

KGBruce said:

Glad to be able to bounce this back & forth!! The cable that mounts to the axle is on a spring. So what happens when there would be a "hop" or rise in the rear suspension is this mounting spring stretches which allows the cable to move upward in unison with the valve so no change in valve action occurs. Simple but clever design! I'd really be curious to see the position of anyone else's valve/cable/spring setting. Anyway, the craziness here is my spring brakes applied on flat smooth ground when I was not applying the brakes.
I checked out the air brake troubleshooting link you posted. Really, really appreciate your help!!

Click to expand...

Still reading myself. From what I see the load sensing goes to the relay valve. Now it would not seem that the load sensing could effect the spring brake system? Are you sure the front glad hand on the passenger side is open and not venting air back. If I remember something vents up to that glad hand. Also there is a one way valve the goes to the forward valves and should never leak back to the glad hand. If that one way valve lets the air back into the glad hand it could apply the brakes? You could just unhook the glad hand for now and or test the one way valve. This is the one on the passenger side under the grill work. Sometime or another seems like you will have to readjust the load sensing valve to work correct. Hope you get it figured. Ask me why I like the 809 series trucks.

coachgeo said:

until one gets this sorted out.... could not the load sensing valve just be bypassed?

Click to expand...

any more thoughts on this? As in what will/could happen if the cable is disconnected temporarily?