Daisy 717 mid-pump hiss

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Hello there, I'm a new poster although I have been lurking in the shadows for a while

I'm also quite new to airgunning in general, and I've just come into possession of my Granddad's old gear. It all seems to be in good to excellent condition, but hasn't been touched for at least ten years so I'm guessing it will need a bit of TLC to maximise performance.

One of the guns is a Daisy Powerline 717 in .177, which I really enjoy shooting and it seems to be very accurate even in my newbie hands. The lever was originally poorly adjusted so that there was no resistance whatsoever when actuating the pump lever before cocking the bolt. This meant that the lever flapped around a bit when un-pumped and I believe also means the gun was operating at less than maximum power. I've now adjusted it as per the instruction manual. However, after about 50 shots it has developed a slight hiss during the pump stroke, around the point of maximum resistance. This stops once the pump stroke is complete however, and it seems to hold pressure ok.

I'm assuming this means that I'll need to replace at least one seal, but I could be wrong. Can anyone point me in the right direction, or is this actually normal? It only seemed to start happening after I adjusted the pump lever, but I made sure to stay within the specs.