Avoid dieseling after cylinder resealing

I opened a new thread to not take over the thread of @matethegreater concerning Hydraulic cylinder disassembly On my big Luna forklift I have to reseal the 2 side lifting cylinders. The free lift cylinder is already resealed. To be honest I didn't know about dieseling before so I resealed the tilting cylinders and reinstalled them on both ends before applying Hydraulic pressure. Sure no load on the forks but the weight of the mast and carriage. After thinking about how to reinstall the 2 lifting cylinders I only see two possibilities to it to avoid dieseling.

1. Lift the inner mast with carriage to full height to make space for the side cylinders freely. I don't know the weight so I'm not sure if my 5ton forklift is strong enough. I have to bigger ones but the lifting height is not sufficient.
2. If that does not work I can mount 2 big flat bars with 2 holes to move the 2 cylinders outwards on both sides. Both solutions do not place the cylinders horizontal and also not the rods in middle stroke but I don't see a good way to do that. Does anyone have a better idea from experience with big forklifts or machinery?

@Nige I would like to continue here if that's okay. I read your thread about the assembly of the cats and the Hitachi excavator in south America. How is the assembly of the cylinders done in this case ? cylinders prefilled with oil? But then still some air after connecting

Cat cylinders weren’t prefilled it was always move cylinder slowly and don’t bottom the cylinder when they were first installed

Fork lift cylinders are usually one way. There will be a little bit of film on the rod side of the piston and a small line to vent any air and leaking oil back to the tank. Just work the lift slow part way and then slowly lower it moving higher each time you cycle it.

I don't know about your machine, but most straight mast forklifts use single acting cylinder for mast lift, double acting isn't necessary as the weight of the mast, carriage and forks will retract the cylinders. And you typically find two types of cylinders. 1) doesn't use a piston seal. When you pump oil into the base of the cylinder, displacement makes the rod go out, no piston seal needed. Those cylinders typically have a bleeder screw just below the cylinder gland to bleed air. And 2) On your machine, looking at the pics, I can clearly see a pipe connected just below the gland. That tells me you machine has a piston seal. So as the rod extends, what's on the rod side of the piston has to vent, that's what the pipe is, that line is connected to a tank line. It not only provides a vent, but it also prevents building of pressure on the rod side of the piston were the piston seal to fail. With all that said, on your machine, after you rebuild a cylinder and install it, simply hook up the lines and extend it, it will purge the air from the rod side, then once you lower the forks it will draw in oil for the return line and fill the rod side of the piston. Several cycles will finish this. But check your main cylinder in the middle, see if it has a bleeder screw. If it does, raise the forks just a few inches and open that bleeder screw until all air is exhausted.

I know that normally the lifting cylinders are single acting. but here it is strange. it has one line coming from the main valve block . it goes to the 3 cylinders. but the line which comes down from the top of the cylinder near the gland goes back to the same inlet.

the small hose in the background of the valve comes down from above but high pressure I assume. hoses were changed since I took the picture. I never saw that system before. the forklift is a Spanish machine, Luna was mainly a Crane manufacturer in Spain but also built forklift from 8-52to Capacitiy. this one is 20to Capacity

willie59 said: I don't know about your machine, but most straight mast forklifts use single acting cylinder for mast lift, double acting isn't necessary as the weight of the mast, carriage and forks will retract the cylinders. And you typically find two types of cylinders. 1) doesn't use a piston seal. When you pump oil into the base of the cylinder, displacement makes the rod go out, no piston seal needed. Those cylinders typically have a bleeder screw just below the cylinder gland to bleed air. And 2) On your machine, looking at the pics, I can clearly see a pipe connected just below the gland. That tells me you machine has a piston seal. So as the rod extends, what's on the rod side of the piston has to vent, that's what the pipe is, that line is connected to a tank line. It not only provides a vent, but it also prevents building of pressure on the rod side of the piston were the piston seal to fail. With all that said, on your machine, after you rebuild a cylinder and install it, simply hook up the lines and extend it, it will purge the air from the rod side, then once you lower the forks it will draw in oil for the return line and fill the rod side of the piston. Several cycles will finish this. But check your main cylinder in the middle, see if it has a bleeder screw. If it does, raise the forks just a few inches and open that bleeder screw until all air is exhausted. Click to expand... there is no bleeder screw on the free lift cylinder. so you think it is not a Problem to reinstall the cylinder and lift it slowly without load on the forks but with the weight of the inner mast and the Carriage and forks ?

I'm thinking that design will purge the air from the cylinders after it's been fully cycled a few times. That round rusty part that the hoses are attached to is likely a velocity fuse. A velocity fuse allows fluid to flow back out of it (lowering forks) at a certain rate. But since the cylinders(s) have no holding valves, if you were to blow a hose the fork mast and carriage would come crashing down rapidly. If I hose blew, that sudden rush of oil out of the busted hose would trip the velocity fuse into a position internally that restricts flow making the mast and carriage come down at a reduced speed instead of crashing down.

Ok that makes sense