leak in rear air cylinders

i have a leak somewhere in the rear end of my car i have a 96 chevy cavalier fully shaved and has air ride i have cylinders on rear and strut bags on the front ..........when i hit the switch to raise the back end up it goes up and stays up for about 20 seconds and the it slowly starts to leak out and after about a minute it is all the way back to the floor........i have checked the hoses and fittings with water and soap in a bottle no leaks there.valves are good also.what i was thinking was maybee the cylinders themselves have a leak in them and i will have to buy one of the cylinder rebuild kits..........i was just wondering if anyone else has had this problem before and i need all the help anyone can give me thanks!!!!