I have an older .20 Sheridan pump that my grandfather just gave to me that he said has sat in his gun cabinet over probably the last ten years or so he said it had pressure when it was put away but wouldn't build pressure when I got it so I lubed with pell-gun oil and worked the pump a bit and still nothing so I left it sit a few days then read about cocking the bolt then pumping so tried that and it worked then so I thought I was in the clear then I went to pump it the next day after it sat in my car in the cold for a few hours and again back to no pressure then once in heated area it was working properly again. Anyone have an idea what it could be? Was going to get a rebuild kit from the start but I'm not exactly sure of the year and all. It has a serial of 241547 and says c series I'm pretty sure from my research it is a 1978 but not positive. Any help is appreciated thanks