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I just picked up a Watsco 4 cfm vacuum pump. From what was told it was brand new. It was purchased by an old guy some years ago and it set in his shop. He retired and has now pasted away. The pump is clean and looks like it has never been touched. There was no oil in it. So I took it home filled it with oil and started it up. It will pull a really strong vacuum for about two or three minutes, Then as it gets warm it will pull less vacuum then stop. it is getting too tight for the motor to turn it. I drained it and pulled the tank off. I pulled the side of the pump off. It was clean inside and all looked brand new. I couldn't open it any further because the impeller I think is pressed on and I didn't want to damage the pump buy trying to pull it off to see the other side of the pump. I put it back together and same thing. It stops as soon as it warms up. Any ideas as to why this is happening and how to fix it?