Hydraulic lift system Hydraulic, mechanical and electrical components of the lift system
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Old 07-17-2017, 04:00 PM   #1
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Default How to remove and replace the hydraulic lift cylinder.

It was suggested that we put this on as a topic in the lift system section. Thanks to Treeclimber for the write-up.

Hi! I've done three cylinder remove/replaces. Basically, with the trailer lowered, a tarp on the ground/cement--take several pictures of pulley placement, locking bar and guide rod placement, etc. I then blue taped the Front Lower cable "FL" same for Rear Lower "RL". Your piston should be almost fully retracted. I use a plastic wire tie(zip tie, medium duty to tie the cables as tight around the pulleys as I can, remove the bolt and washer and again zip tie them out of the way, try to keep SOME tension on them. CAUTION: the locking bar is not held in with any fasteners, it MAY fall after the next step. Ask me how I know. There is a taper type pin on the guide rod-remove that pin SAVE IT! Slide the guide rod to the street side until the curb side falls out. Once free of the HUGE plate, put the pin back in. Using two open end wrenches, remove the locking nut , then the retainer nut from the piston end. AGAIN CAUTION! the Heavy plate will come down (FAST) upon removal from the piston rod end! AND that locking bar!
Next- street side, This area is not the easy end because of space and ingenuity is needed. Carefully remove the "B" nut on the hydraulic line, cap it or use a zip lock bag to prevent contamination. Keep it out of the way without kinking it. Remove the plastic vent line and allow it to drain into a container. ALSO remove that stainless adapter where the "B" nut was attached. On the piston end, remove the cotter pin, nut, washer and then with what ever safe method you can, remove the large pin and the piston should come free. Safely remove it from the trailer.
When you have it repacked ensure you let then know that it is filled with Dextron 1 thru 6 Automatic transmission fluid-- not a hydraulic fluid. Should be bench tested to 2600 psi.
My preferred shop caps and tapes the piston mounting areas, sand blasts it with 220 grit, re-builds it, tests it to 2600psi then primes and paints it gray. Re-install in reverse order. When you raise it, only go about 3-6" several times to get any trapped air out. Then raise and adjust cables if necessary.
I assure you it's not hard, just makes your brain work (or get soft). Take your time, you'll see how it needs to be done.

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Here is an added note from Jim L concerning transmission fluid:

I want to raise a flag about something treeclimber said in his post. He stated that if the hyd ram was resealed to use dextron 1 to 6 to fill the system. I have in the past thought this was correct also. I have a allison tranny in my silverado an I just started following the Transmission section of the duramax forum. One of the experts who post on that forum was the fluid engineer for Allison that wrote the specification for the transmission fluids approved for use by Allison says that Dextron 6 is not compatable with the earlier Dextron fluids. In 2006 Allison put new types of seals in their transmissions that are compatable with Dextron 6 but Dextron 3 is to be used in transmissions built before 2006. They found out that Dextron 6 that had been put in pre 2006 transmissions caused the seals to harden and caused leaks. For anyone wanting to research this further go to the duramax forum and the oils and fluid section of that forum. The engineer's handle is hzjcm8 and you can search for his post. There are 98 pages on just the transmission fluid subject.
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Old 07-20-2017, 11:21 AM   #2
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Addendum: Reason for removing the fitting from the cylinder after removing the "B" nut is to allow the cylinder to clear the hole in the frame. Thanks jedelen for questioning that step.
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Old 07-20-2017, 02:12 PM   #3
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hey do u still have a pic of the lift lock i replaced my cylinder with a differant one and cant find where the lock holds it up under neath
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Old 07-20-2017, 03:13 PM   #4
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d0w0r, I have no idea if your safety bar is like mine, but we'll try. Every bar I've seen has three "stops", that's what I call them. If I have the pics right, the first is the curb side. I fits loosely in a square hole, and in my case the cable goes underneath. The middle stop is the actual stop for the raised top, it is on the CABLE side of head block (the HEAVY steel block). And the third can be seen at the end of the cylinder. Ya with me? Hope the pics help. The problem is, there is NO way that I know of to install the safety bar without removing the guide rod. The safety bar slips into a pocket above the cylinder, then is slid into the curb side pocket and the cylinder holds it in place, while you re-insert the guide rod. In each pic you'll notice each stop. The bar MUST be in that order to work right. The stop above the cylinder prevents the bar from going too far to the street side, thus preventing it from falling out.
Let us know how it works out.
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Old 07-20-2017, 03:18 PM   #5
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I may add that when you pull the cable inside, your raising the center stop ABOVE the cable block, thus freeing the piston to retract. So you need only hold the cable a few seconds of the lowering cycle.
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Old 07-20-2017, 04:20 PM   #6
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Tree, I think the BEST approach would be to get under the trailer and watch how things work while someone else activates the safety bar (and possibly the lifting mechanism) BEFORE trying any repair to any of this. Of course, if it's already apart, this is a moot point.

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Old 07-21-2017, 07:52 AM   #7
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Jack, in this case the bar is Not installed. The top could, of course, be cycled, but the bar cannot safely be installed while up. I, mentally, (could be a problem!) went through each step of r/r of the cylinder and vividly remember the bar clunking me in the head! Without removing the guide rail, I don't see how it could be done. Unless you cut a big hole in the floor and install from the top. NOT!
I seriously recommend each owner, crawl under and watch the evolution of the cycling of the top, up AND down, NO beer prior please!
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Old 04-26-2019, 07:47 AM   #8
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Default Heavy plate.

I will be removing my cylinder soon and have been reading the forum for advice, in the first post its mentioned that there is a nut holding on the plate, mine looks to be threaded directly to the plate, is this normal? I am working on a 1977 model.
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Old 04-26-2019, 04:43 PM   #9
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Tug, mine was also threaded to the plate and for some reason a nut capped off the threaded portion. I guessed those extra threads (about 1/2") were for some adjustment. I counted the exposed threads and replaced with the same count. I still can feel the safety bar! Again I caution all about falling safety bars. As you'll find out, it's not hard removing the cylinder, just take your time and get creative with your tools. My hyd shop had it rebuilt and tested to 2600 psi in a short time. Let them know to test it with Dextron 3, 4, or 5 tranny fluid.
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