How do you fix old hydraulic things on the back glass of an SUV?

[JaketAKACaptainCrasht]

I have a 1991 Blazer, and to access the back, you have to raise up the glass, then open the tailgate. But the things that hold the glass up only work every now and then. We've replaced them with others, but they're the same. What can I do to make them work again? When raising the glass, they actually hold it down and make it difficult to raise, and then it just falls once you open it.