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Here`s what my book says. 1. Improper tyre pressure. 2. Loose pump drive belt. 3. Low fluid. 4. Bent or poorly lubed front end parts. 5. Improper front end alignment, especially caster. 6. Bind in steering column. 7. Air in hyd. system (unlikely). 8. Low output or leaks in system. 9. Obstruction in lines. 10. Pump valves sticking or out of adjustment. If everything else checks out I would say the pump.