M936 wrecker boom leaking down

I have about ran out of things to replace . Truck is in great shape other then the boom will not hold its own weight. Bought the truck 2 year ago knowing that thinking easy fix. Well not really. Bought new old valve control in wooden crate, installed whole complete valve control,not fun but sure it will fix it . Nope does exact same thing. Raise it all the way up with just the boom weight and it will be bottomed out in 30 seconds. So found 2 new old stock boom cylinders in crates. Installed them , does the exact same thing did not change nothing, not happy lol .
Any ideas , on the bottom of each cylinder there is a snubbed , could it be that?

Just because you got them NOS doesn't mean the seals in it are any good. Only thing I can think of to try is to put a valve in the lines from the cylinders to the valve body (lower lines) and raise the boom, then close the valve. If it still lowers, issue is in the cylinders. If it doesn't issue is in the valve body.

Thanks was thinking of trying something like that next but thought I was missing something. Control valve and cylinders I bought looked excellent sealed up with some oil in them. Just crazy that it never changed the leak down.

colesmotorsports said:

I have about ran out of things to replace . Truck is in great shape other then the boom will not hold its own weight. Bought the truck 2 year ago knowing that thinking easy fix. Well not really. Bought new old valve control in wooden crate, installed whole complete valve control,not fun but sure it will fix it . Nope does exact same thing. Raise it all the way up with just the boom weight and it will be bottomed out in 30 seconds. So found 2 new old stock boom cylinders in crates. Installed them , does the exact same thing did not change nothing, not happy lol .
Any ideas , on the bottom of each cylinder there is a snubbed , could it be that?

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I have to agree with Ferroequinologist .

Just because you have two New Old Stock (NOS) pieces of hardware doesn't mean there aren't problems with the seals on the lift cylinders. It does seem odd that both of them would fail at the same time immediately after installation though. The valvebody might have problems with rust that may have scored the internals - OR - that valvebody may have been returned "new defective" and after it sat around for ten years, to oil drained out of it... Just guessing there.

If you do a little "rigging" you could add a pair of ball valves as close as possible to the cylinders. Raise the boom, then reach over and turn off the the fluid valves that you installed. That should stop the boom from falling - IF - the packing in the cylinders is good.

That doesn't FIX the problem, but it will confirm that if the boom still drops that there is a problem with the cylinder packing. You will have isolated everything else. If it leaks down, the fluid is bypassing the rubber o-rings inside.

Whatever you do, DO NOT forget to pull on the UP valve. Chances are good that you will break a hose if you do.

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The tube that runs up the side of the cylinder could allow fluid to allow a small amount of drift. Pressure will eventually equalize on both ends of the piston if you install a pair of ball valves at the base of the cylinder - but eventually it has to stop coming down - even if there was no packing in the cylinder. That eliminates potential valve leakage.

Both cylinders are tied together, so if one is bad both will leak down. Fixed a guys dozer with that exact issue. Blade would leak down, but only one cylinder was bad. Isolated each independently with a valve, and caught the leaker.

There's just not much to that system, it is either in the valve body or cylinder.

I know it's a long shot but trace the lines and make sure it is plumbed correctly.

Any debris in the oil when you changed the cylinders? Doesn't take much to ruin a set of seals.

Thank you much, will update when I get valves installed

Well I put a ball valve between the control valve body and the tee fitting the goes to the bottom hoses on the cylinders. Thought finally , well not finally, raised boom up and down about ten times to get air out. Raised it up shut the valve and does exact the same thing. Did not help at all. So now it must be cylinders leaking to the return ???? Do I just put valves off the upper hoses on cylinders now???

The issue is definitely in one or maybe both cylinders then. Even with valves on both ends, a cylinder can leak by internally. You could go through all the hassle to try to find which cylinder it is, but I don't feel it's worth the lost fluid and frustration. If it were me I'd pull both and take them to a shop and have them checked/repaired. At this point even if you isolated it to just one, I'd be wary of the other.

Actually didn't you say you swapped these? Take the ones you took off to the shop. Ask them to test them.
I know I might be but you did verify all the plumbing is correct according to the TM right?

Yes plumbing is correct, rebuilding other cylinders sounds like good idea. Would you know or anyone else know where I could buy new packing for cylinders ?? I kind like doing the rebuilding myself, these are pretty easy cylinders. Appreciate the advice and help , thanks a bunch!!!!!

colesmotorsports said:

Yes plumbing is correct, rebuilding other cylinders sounds like good idea. Would you know or anyone else know where I could buy new packing for cylinders ?? I kind like doing the rebuilding myself, these are pretty easy cylinders. Appreciate the advice and help , thanks a bunch!!!!!

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My opinon FWIW is anything you could find in the military warehouses for these hydraulic cylinders will be old and tired. Most (not all) warehouses are not environmentally controlled and rubber and nylon will have already deteriorated.

All the NSN parts are listed in the TM. A picture might be useful to the hydraulic shop. That might help them know what they should expect to see when they crack open the cylinders... I would think they should be able to measure and match "like parts" for a rebuild.

True I'll probably just to hydraulic shop here, thank you

Surplus center has the packing. Maybe even a surplus replacement cylinder for cheap

I only paid $500 for each cylinder plus $450 freight for ones I bought . Looked and thought where NOS , but like they say the packing gets old .

A couple of years ago GL had a 936 boom up for bid, I put a $100 on it and forgot about the bid, a few days later I won! When I showed up in OKC to pick it up it had two new lift cylinders packed inside the boom! Never know what you will get, well most of the time the shaft but sometimes not!

rumplecat said:

A couple of years ago GL had a 936 boom up for bid, I put a $100 on it and forgot about the bid, a few days later I won! When I showed up in OKC to pick it up it had two new lift cylinders packed inside the boom! Never know what you will get, well most of the time the shaft but sometimes not!

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We only get the shaft from GP now. We all miss GL

Cool, dumb question, GL ,GP ?
I kind of like messing with these trucks kind of a goof hobby. Also have a M931A1 and Hemtt M978. Wife thinks I'm nuts and I don't argue the fact , that way when I buy something ridiculous I just say , I'm sorry I'm nuts won't do it again lol

Gov liquidation the old surplus get stuff for cheap seller and gov planet the start the bid at full retail seller

colesmotorsports said:

Yes plumbing is correct, rebuilding other cylinders sounds like good idea. Would you know or anyone else know where I could buy new packing for cylinders ?? I kind like doing the rebuilding myself, these are pretty easy cylinders. Appreciate the advice and help , thanks a bunch!!!!!

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Most any hyd shop worth an ounce of salt can take the measurements of the piston and get you the correct packings.

Plus there are books out there for hydraulics that you can look you sizes up and it tells you the o ring and retainers required for said measurements

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Talking about GP:

I have seen this discussed before, but it is really interesting that my most recently purchased thing included an invoice. That invoice doesn't show the 2.95% credit card fee on that invoice. That is interesting. Funny that the credit card companies won't allow that behavior.

Yes, I know it costs them and yes, it could have cost me nothing with a wire transfer. Just interesting...
Next time I will most likely do a wire transfer to save those dollars.

They seem just a bit crooked - with evidence - to me.

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colesmotorsports said:

Yes plumbing is correct, rebuilding other cylinders sounds like good idea. Would you know or anyone else know where I could buy new packing for cylinders ?? I kind like doing the rebuilding myself, these are pretty easy cylinders. Appreciate the advice and help , thanks a bunch!!!!!

Click to expand...

I usually take the cylinder apart, and take the packings into the local hydro shop. They had all the seals for that 1968 Allis Chalmers dozer cylinders in stock. Take your new seals home and install. Easy peasy.

Had a shop quote me $285 to rebuild a cylinder once. New one was $175. Rebuilt it myself in an hour with $12 in seals...