Critical FMTV PM not covered in TMs

[NDT]

I was in the process of raising the cab of the M1078, when just starting on the way up, the cab latch cylinder blew out, spraying hydraulic fluid all over the right side of the truck. The oil came from the indicator plunger, which we are supposed to look at to make sure the cab is latched down. Needless to say, I could no longer raise the cab. Fortunately I was at the shop, working out the zillions of other less catastrophic bugs these trucks seem to be plagued with. I would hate to think if I was on the side of the road somewhere, needing to check the oil or something. Anyway, it is a simple repair. The TMs only cover removing the latch cylinder assy, but this is not required. Get your 1 3/4 socket, open end wrench, or maybe even channellocks, and remove the aluminum cylinder end cap. Inside is the piston, which you can pull out with your fingers. On it you will find a 1 1/8" x 1/8" O-ring that is very brittle. O'Reilly's had a National p/n 216 O-ring which worked perfectly. $2.50. Problem solved. Picture shows the piston, the cylinder end cap, the broken O-ring, and the socket I used.

10 year later edit: The other o ring is a P16 metric o-ring. This is the one on the plunger shaft.

 

[DSD277]

That was one of the many repairs needed when I was putting the LVAD back together. It is actually the latch's lock. As pressure from the hydraulic builds up, it slides the rod out of the latch releasing the latch to open, the rod on the other side is a visual indicator to the latch being locked.

If need be, the strike on the back of the cab can be unbolted to allow the cab to tilt. 3 bolts across the top of the strike, 2 bolts WITH nuts at the bottom.

 

[bkfj40]

A few follow up questions to this as I need to replace the oring in my newly acquired 1996 m1078

where is additional fluid addenda? What type of fluid?

how do you bleed the system?

 

[NDT]

You add hydraulic fluid (jack oil at the auto parts store) in the air-hydraulic power pack filler port between the spare tire and air cleaner. The system self-bleeds after the repair. Welcome and good luck.

 

[shadowrwolf]

good to know mine is leaking bad

 

[Wire Fox]

Good info, guys. Thanks for this. I'm sure this is a problem I can expect to encounter some day.

 

[Awesomeness]

Thanks. Mine is leaking bad and won't latch the cab and more. Off to find that 216 oring...

 

[DanM7890]

Just an FYI, Sears has the sockets in that size and they are having a special on them. I paid under $10 for it.

 

[Awesomeness]

Just an FYI, Sears has the sockets in that size and they are having a special on them. I paid under $10 for it.

What size drive? 1/2"? 3/4"?

 

[DanM7890]

What size drive? 1/2"? 3/4"?

3/4" than went to Lowes for the adapter.

 

[LFGeorge]

Same part different question. What am I supposed to do if the lock pin doesn't lock. Tried tapping it, but it seems solidly set in place in the open position. Thanks

 

[Awesomeness]

Same part different question. What am I supposed to do if the lock pin doesn't lock. Tried tapping it, but it seems solidly set in place in the open position. Thanks

I was just posting with the same problem! I changed the o-ring, and the leaking has stopped. But the lock pin no longer engages when the cab goes down. Here's what I've tried so far...

1. With the cab up, I can use a screwdriver to depress the latch cam, but the lock lever is held open.

2. With the cab up, I can use a screwdriver to depress the latch cam, and then use a clamp to force the lock lever pin to close [into the cam]. After that, it maintains pressure on the cam, and I can use the screwdriver to pry the lock pin back and release the cam, and when I push the cam back down the lock pin snaps into place.

3. With the cab up, I repeat step 2, and use a clamp to force the pin into action. Then I pry the pin out and release the cam, with the pin still keeping pressure (wanting to snap in place in the cam). Then I lower the cab, and the pin will NOT snap into place. I even climbed behind the cab, with it down, and tried to use the clamp to force it in, but it won't go.

Without knowing more about the system, I'm assuming that the pin is supposed to spring load itself into the lock position, and that when you raise the cab it pressurises the line to the pin to force-hold it open while the cab raises, then it's supposed to release the pressure when lowering the cab so that once the cab is in place the pin will spring into lock position again on its own. So this makes me think that the pressure is never being released on the line, so it's being held open even when the cab is down. Thoughts?

Is the lock just an added safety precaution, or is it what is doing ALL the holding down of the cab while driving? So it is totally unsafe to drive unless the cab can latch?

 

[DSD277]

When you go to raise the cab, the initial pressure unlatches the safety and then the pressure raises the cab. When the cab comes down, the lack of pressure allows the safety latch out to catch the hook.
More than likely, the piston has frozen from water and and crap in the cylinder. Remove the cylinder cap and clean everything inside. It's been years since I did mine, but iirc, there is a return spring inside the piston cover.
Basically like an old VW bus 1/2 brake cylinder , but a whole lot larger.

 

[Awesomeness]

Yes, all that is true, but I just had it all apart to replace the o-ring, and as my steps 1-3 above describe, the latch pin is at least capable of moving. Everything inside looked nice... shiny, clean, no rust or corrosion.

 

[NDT]

Assuming the pin is free, you have to keep pumping quite a while after the cab is down, finally the pin will retract.

 

[Awesomeness]

Once the cab is all the way down it pumps maybe 10 more times and won't pump anymore. If you release the button and pump again, sometimes you'll get a couple more clicks. Is that the expected behavior?

 

[NDT]

Once the cab is all the way down it pumps maybe 10 more times and won't pump anymore. If you release the button and pump again, sometimes you'll get a couple more clicks. Is that the expected behavior?

Yes.

 

[Awesomeness]

Is the lock just an added safety precaution, or is it what is doing ALL the holding down of the cab while driving? So it is totally unsafe to drive unless the cab can latch?

 

[tennmogger]

For those doing repair on the latching cylinder for the first time, please be safe. If you raise the cab expecting to work on the latching cylinder, BEWARE, the cab will lose hydraulic pressure and drop if the lock is disconnected or disassembled in place. It becomes a huge mouse trap with you as the mouse.

I worked on mine from the bed of the truck with cab down. It's a little awkward but I felt safe.

(yes, the latching cylinder is ALL that holds the cab down.)

Bob

 

[DSD277]

I rarely pump the cab all the way down. Once it's over center, I just let gravity bring it all the way down. It should self latch
I can say for a fact the cab will pivot forward if you slam on the brakes while moving.