Where to start troubleshooting air bag suspension system

back end of the car almost sitting on the ground? friend of mine has one of these cars.it was his fathers car,father passed away and son wanted to keep the car and fix it,dad bought it new.he was in the process of replacing/buying the parts for the rear suspension. struts,bags,pump? he found a little place near here that told him to get rid of that crap and buy the conversion kit.so he did,saved him a ton of $$$$$.no more problems,he's happy now.

We put. New gas tank new shocks. And new Air struts. In and only one. Will. Fill up ? How do. We fix it