the jack extends because the fluid heats up and expands. if this is a new set of levelers, i would say that a hose had been run near a heat source. but its several years old, so i would say the springs are getting weak because of rust or road grime. you can either clean them with a wire brush or rust solvent, but if it was mine, i would replace them.
you can get them from hwh, stuarts in elkhart in, or on the internet.
the fluid doesnt have to be changed unless you have allowed it to be contaminated. it doesnt get very hot, it doesnt wear out and doesnt evaporate. it also has a 30 micron sock filter in the tank, in case something get into the tank while filling it.
if you dont have a visible leak, checking it once or twice a year is ok.
i work for hwh.i do not sell parts or solicit service work on this forum.
i do diagnostics