Recently acquired a JD 2640 with a Kelly 800 loader that has hydraulic problems. Changed the oil, cleaned the filters, and got the air out of the system. Hydraulics work great at startup at high RPM only, but as soon as you drop the RPM hydraulics start shuddering or fail totally. I changed the oil, cleaned the filter, and the results are same. My grandpa suggested that the hydraulic pump might be worn out. He said that the oil is pumped by paddles with little cups in them. He said that with age the cups wear out, which means that the pump can't pump oil at idle speed and will only be able to spin fast enough to move enough oil to move the loader at high RPM. Is this possible? If so, can i replace the pump? If so where is it located? Any help will be appreciated. Thanks.