Welcome to JUST ANSWER heavy equipment. With years of experience, I look forward to doing my best to help.
Model number? Which cylinder? Typical cylinder you have to loosen the gland nut, remove the gland, pull the piston from the barrel, loosen the nut holding on the piston, remove the piston so the gland can be removed from the rod. Then the new seals installed in the gland and on the piston and then reassembled.
are check valve orings high pressure? seal seems simple enough!
O'rings are made in different durometers, which is hardness of the o'ring. The check valve o'rings need to be better than a standard o'ring and are high pressure o'rings. The harder the o'ring, the more pressure it can handle.