Do you have a greenhouse roof vent that is no longer opening or not opening enough?
Over the years, the piston moving through the seals gradually wears the seal material away, allowing a gradual seepage of wax out of the cylinder. This reduces the pressure within the cylinder and needs to be replaced. But it could also be that the wax in the cylinder just needs a simple kick start and a warm bath. Here is a tip to see if you can get them working again: