It is not proper for any cylinder to leak down. Common, yes, on cylinders with worn seals. Whether to repair or not depends on your level of comfort with it the way it is. There is drift tests that the manufacturer should give details on in the service manual. I would bet that yours would fail, so I wouldn't bother.
What happened is that the piston seals are worn. While your cylinder sits idle, whichever way it is being pulled, fluid slowly leaks past the internal seals and away it goes. The worn piston seals will also give a drop in available cylinder power. Nothing inside the cylinder should be damaged if you don't repair. Most pistons in this application have wear bands on either side of the seal. What you may notice is a drop in cylinder power.
