Need help identifying an older Sheridan

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
 
Pics would help-If it has the rocker type safety its a newer model, the older ones had a "hold down' type of safety.
I got a kit for an 80's era gun and started by replacing the pump cup and cleaning the gunk out of the pump tube. I lubed the cup and tube with Pellgun oil and after some pumping and dry firing it now holds air. I never did tear int o the valve on that one. If you do get a kit make sure you have the valve tool as well and do search for 'how to rebuild a Sheridan model c'. 
Here's an '80s era rocker type-front sight was broken off when I got it.
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Looks like you're correct on the year. 1978 serial numbers 211021 thru 257460
Put two pumps in it and let sit over night. If it fires the next day then you can probably get away with only replacing the pump cup on the front end. Make sure you clean the gunk out of the cylinder if you do it yourself. Mac1 or JG airguns carry parts.
Post that pic

Steven
 
I had one of these a few years ago I bought used. Same symptoms. Open the pumping handle and take some CLP lube
and put about two good squirts into the hole that sucks the air into the chamber. Hold the barrel straight up while
pumping the gun up about 8 to 10 pumps. Let is sit for about ten minutes. Then shoot a pellet out and repeat this
a few times. THEN store it over night with three pumps in the chamber WITHOUT the bolt cocked...Let me know
how this works....AK
 
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