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Here are the trouble shooting pages out of the Oliver Manual HYDRAULIC UNIT WILL NOT LIFT
1. Oil level low-Check fo roil leaks and add oil t proper level 2. Unit raises in Postion Control, but not in Draft Control-Adjust internal and external draft control linkage. 3. Pump or pump drive failure-Check the oil flow by removing the hydraulic system drain plug and starting the engine. If little or no oil flows, repair the pump or pump drive gears. If the tractor is equipped with an external valve check the oil flow from this valve. Also check the left capacity of the remote cylinder. 4. Dirty or faulty internal valve- Disassemble, inspect and clean all parts thoroughly. Look for dirt or small particles of metal under the relief valve ball located inside the hydrostat spool bore in the body and replace both parts if either one is damaged. 5. Bent or broken internal control linkage- Compare suspected parts with new ones and replace if required. 6. Oil leaks-Check all "O" rings and pipe plugs in the housing, cylinder and valve. 7. Broken cylindr or housing- Check hydraulic passages in the cylinder and housing for cracks or broken sections. 8. Dirt under the ball in the external valve relief valve- Dirt under this vavle will prevent the three point hitch from lifting. Dissasemble and clean valve. 9. Mechanical failure of the lifting linkage- Examine piston, cylinder, rocker arm and cross shaft. 10. Pump not primed- Move control lever to extreme down position and operate engine at high idle speed for two or three minutes. There hope that helps you pinpoint your proble
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