I can’t help, but believe I have a similar problem with an old model. Mine is a 20th anniversary model 880, so I’d guess it was acquired in about 1995. I left it pumped up, too, and it was hot. Now the pump handle not only does not pump up pressure, but whatever valve is supposed to let air from pump into chamber seemingly failed, so now the pump handle is sucked by negative pressure back to the closed position. I’m debating whether to fix or not, because a new like gun is about $50, and parts needed might approach that. It was very accurate, though, so hate to just chuck it, and I probably only fired 50-75 rounds through it max (I’m of retirement age, bought it when I lived in a city).