just did mine on my l555 and my model did not have a glan nut but rather the head was secured in by a retaining snap ring you actually need to force the head or gland or what ever you want to call it back into the cylinder then remove the retaining ring then slowly charge the cylinder and the head comes out the packing on the end is secured with a nut your new holland dealer sells a whole seal kit for about 70 bucks but you might get lucky and just replace the bad seall l there is about 6 differnt seals in total i bought the kit and works gread comes with a new pston for the end of the ram that holds all the seals it a little extra expense but well worth the results keep us posted on your fix
sorry for the no punctuation i apologise. but never the less good luck.
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I did my bucket cylinders once and they are waiting on the job again. It's an older Daewoo but this time I polished the piston and the internal surface of the cylinder with a flex extension on my drill press. For the piston I used an 8" buffing wheel on my bench grinder/polisher I used back in my knife sharpening days. All my seals came from Doosan. Take a good picture of your setup before you remove any old seals. I remember only one white thin seal was the most difficult but I soaked it in hot water to help the expansion/flexing and was finally able to persuade it to slip in place without ripping it. Like many posts cleanliness is your best friend! (No grit) my final stage before reassembly will be a very clean bath in an aqueas parts washer. The past job was a shade tree exercise under Texas Oaks that lasted over 10 years! No polishing and no parts washer!