Just saw your video- looks like your hoist cyl is air bound. This could explain the extreme lowering speed as well. When the forks are fully lowered the hyd tank should be full- can you physically see the oil in the tank? If so- is it foamy looking? If it's foamy- it's either the packing on the hoist cyl s*cking air while lowering (not probable but possible) or there's an issue with your hyd pump on the suction side- either a bad connection in the hosing or the pump itself is bad- check the tank & post your findings

Got your email- your lift is a GLC 030 or 040 depending on the lifting capacity & was built in the mid '70's. Can't tell from your pic the upright that's installed- another pic from the front is needed.

I can see in your pic the side rollers on the carriage which are in need of service to stop this side to side rocking motion. Some of these older Yales had a hoist cyl which actually wasn't bolted in per say- they were secured in the lift by sliders on the rear most channel of the upright. If these "sliders" are missing/faulty- all kinds of issues with the upright staging correctly, along w/ extreme looseness of the entire assy can occur. Combine these issues w/ the fact that the hoist chain goes under the lower chain sheaves on the hoist cyl which could in fact come off the chain sheaves which in turn will alllow the upright to fall unexpectedly from the chains now being "free"- this was the cause for my concern.

Send multiple pics to my email showing the chain path (with the upright raised to the limit) so I can see if this is the case.

As far as bleeding the hyd lines- if you remove & cap the lines at the hyd control valve- no bleeding will be necessary- you will however have to remove the side shift carriage from the stock carriage also.

I know the side shift reel is leaking quite a bit. I've never had a forklift that wasn't at least 30 years old and leaking somewhere. If I remove the 2 reel lines at the diverter and cap them off do I have to "bleed" air of the system somehow? Wondering if this will help with the main cylinder. We rarely need the side shifter and when we do use it the truck stalls if we're not stepping on the throttle.

Hi BBforks. I will email some pics this morning. I can't find a placard anywhere on this truck to give me a model number.

Look for a allen head bleeder cap screw in the center of the shaft on top of the lift cylinder.

Can you send a pic to my email? Bouncing cylinders, super fast lowering rates & super loose carriages are a recipe for disaster. I'd like to see exactly what kind of lift we're working with as some hoist cyl's aren't actually bolted in place- rather they are sandwiched in place & bouncing isn't something they tolerate well- they can actually fall out of position

Hi, I took a look at the rollers. Still not sure whether is has side thrust adjusters as mentioned above. Will take a closer look tomorrow. My main concern is the ram bouncing. Any way to post a video of it?

When lifting the first ram is smooth, then the next ram bounces as well as the third. When lowering it lowers VERY fast until getting to the last (first?) ram then it lowers slowly. I tried tilting all the way back and raising to full height and lowering completely probably 10 times. Didn't make much of a difference. The Hydraulic res. is full and the fluid seems clean. Have no idea how old it is though. I bought it used.

rocking carriage?
my first thoughts are....
if this is an older unit alot of them had side thrust adjusters inside the carriage roller stub shafts. The way to tell if it has these is to look on the inside vertical bars of the carriage where the rollers are and if you see a hole with a setscrew and nut then there is your side thrust adjustment. These were notoriously wearing out and causing alot of carriage slop and had to be adjusted regularly.
The way to adjust them is to loosen the nut, get a wrench or allen socket on the adjust screw and tight it up snug and keep an eye on where the carriage is centered, keep it close to center but as soon as you feel it touch the channel stop and tighten up the lock nut. When you get to the opposing side to set the max play adjust it the same way , till it just touches the channel and then back it off 1/4 to 1/2 turn and then lock it down.
There may be 4 of these, one on each roller so do them all the same.

If this is a newer truck then it may have side thrust rollers that should be shimmed and also the carriage rollers must not exceed side play clearance specs. If it does then the rollers will need to be reshimmed.
Keep in mind that the channel may be bent causing this also if there has been any impact to it at any time. If you have issues adjusting the rollers or side thrust adjusters then you may have to check it further for warpage or other damage.

all the other suggestions made are also possibilities as well

good luck

Check the upper slide slipper bearings as kevin t suggests, just to make sure whether that could be a factor.
As far as adjusting the chains goes, you need will need to address any other issues (carriage side rollers and carriage main rollers) before doing any adjusting to chain length.
Carriages have two sets of rollers. There are "side rollers" which run PERPENDICULAR to the mast channel, and "main rollers" which run PARALLEL to the mast channel.
The small side rollers run on the web of the channel, and are intended to keep the carriage centered between the mast channels so that the main carriage rollers do not scuff against the flat plain of the web.
The main carriage rollers run on the flange of the mast channel, but very near to the radius where the web and flange meet.
Carriage side rollers are typically mounted on an eccentric axle to allow adjusting the clearance of the side rollers to the mast web.
Main carriage rollers typically use shims/spacers to adjust how they orient regarding their fit/clearance to the channel web.
Your problems be be due to everything, including the mast channels.......being "worn out".
Hopefully you will spot something that can be remedied, without breaking the bank.

You said this carriage has a Sideshifter on it....one other possibility is that the Sideshifter plastic bearings that it rides on is either worn out or one of the plastic bearings is missing completely...causing the Sideshifter to slope and ride on the carriage bar

Hi, thanks for the info. I do mean the Ram/rod. Im going to try raising the mast up and down today. We only have 12' ceilings. Its a double mast and has never been raised more then 10' or so since I've owned it. I think there is a lot of slop from wear. I guess adjusting the threaded chain rods isn't going to help the situation huh.

You can look down into the channel the carriage rollers run in while prying against the top carriage rollers (pry tool between roller and mast channel) to shove the carriage over so you can gauge how much slop there is, and why. Alternate the pry tool from one side of carriage to the other to see he full range of free travel.
When you say " the main cylinder bounces up and down before settling", do you mean the entire cylinder assembly or just the ram/rod part that moves in and out of the cylinder.
I'm pretty sure you mean just the ram.
When a lift cylinder ram bounces up and down when lowering loaded, it indicates there is air trapped within the cylinder.
Air in a lift cylinder may be due to an oil leak on the lift cylinder circuit that allows oil to drain out, and air to enter around the seals at the top of the cylinder while the truck is parked and not in use. So, if the lift circuit has any oil leaks that you have simply been ignoring, that could be the source of air infiltration.
Another way air can become present in hydraulics is if the hydraulic oil is very old and has not been changed in a long time. The oil loses a good deal of its anti-foam additive package and air can become entrained in the oil while the truck is in use.
Later when the truck is parked, the entrained air can come out of the oil and collect in small pockets above the oil.
Trapped air in a lift cylinder can usually be expelled by tilting the mast back as far as it will and then lower the forks carriage completely a few times (raising to to full height and then lowering completely each time).

I will also grant you that mast configurations where there is a single center lift cylinder with a "cross head" chain roller assembly on the ram can be prone to the carriage having much more "see saw" motion than masts having twin lift cylinders with a wider spacing of the carriage chains.

Unfortunately, I'm about a day away from going out of business so hiring anyone isn't an option. I didn't count the threads on the two threaded rods connecting the chains but they seem to be evenly locked in place at both top and bottom of both rods. Maybe an inch of thread showing at both ends. The chains are only 6.5" apart but it would seem I could adjust the right rod to shorten it thereby lifting that fork. I'll look at the rollers to see if i'm missing any but it seems to me that the rollers which are there should have less tolerance between them and the tracks they roll in. I don't know. Its an old forklift. As I mentioned earlier, I can't find a placard to tell me what the model number or age of it is. I bought it from a repair shop last year. The other issue (I think its an issue, never seen one do this before) when lowering the forks with weight on them and stopping, the main cylinder bounces up and down before settling. It's almost like the chains are actually doing all the work. Is this normal?

Your latest description does make it sound like you could have some issues related to chains and/or rollers.
You may even have some rollers missing from the sides of the carriage that you don't even know should be there.
And a side shifting carriage is a bit more prone to exaggerate an unbalanced carriage when shifted to extreme left or right.
It may require the eyes of 2 people to accurately determine where all this excessive sloppiness is coming from.
Might even want to have it inspected by a lift truck professional to find out where you stand with needed repairs.

I'll take a look at that, although, the whole carriage is slanted. I can grab the right fork and lift it up to level. It seems there is a lot of play in the carriage. Maybe the rollers are worn? It's a side shifter. There are two threaded rods connecting the chains. Can I losten the lock nuts and adjust those rods to level the carriage?