I have a B1750 that has low power when lifting the hydraulic lift arms when using a post hole auger. The lift arms will lift the post hole auger up enough to drive around and keep the bit off the ground, but when digging, if the bit is only a few inches into the ground, I can not lift it up. Also when turning the tractor off, the lift arms drop under their own weight. I changed the hydraulic oil, cleaned the filter screens, and replaced the twist on filter. I also pulled the cap on the hydraulic cylinder and pulled the piston and replaced the o-ring and backer on the piston and the o-ring on the cylinder cap. The piston and cylinder looked pretty good- only small surface scratches, so I sanded them with 400 grit emory cloth and cleaned it well. The lift arms still drop when the tractor is switched off. What should I check next?