The pumps are not rebuild-able, a new 11 gpm pump is about 120.00 or so. The shims etc. inside them are to .0001 or there abouts. You really need a pressure gauge to tell if the pump is bad. One thing you can make sure of though is the Lovejoy coupler hasn't sheared a key off or otherwise broken. This would allow the pump to spin until it had a real load, then it would just slip. I had to replace the pump on my Didier about 4 years ago, it had gotten so weak, it would just stall when the oil got hot.