Right, what Randy said. It is a nitrogen system. The problem with refilling them is, where'd it go in the first place? Once they start leaking they need to be replaced as I did with mine a year and a half ago. Mine turned out to be a bunch of micro-leaks so it would take a few days after the recovery from the sag to drop down. In my case, it was the front bags that showed the first symptoms. It was cool the first time I noticed it - I opened the garage and the truck was kneeling before me. LOL!