The rubber bag part of the rear shocks or struts can deteriorate also, and that can cause the pump to continue to stay on, if not that it could be a line, but check the shocks first, if you can get a picture of the rubber part on them, and post it.
If it is neither of those, then as Randall said, it could be the level arm sensor on the rear of the suspension.
If the pump ever totally fails/dies, I'd consider getting shock replacements, they have some with a conversion kit that allows you to manually raise, or lower the rear suspension.
Of course if it is possible, and cost effective, then try to replace whatever you need to so you retain the factor air leveling system, I had a similar problem, and replaced my rear struts, and the problem was solved.
2006 Pontiac Grand Prix GT with SC
1996 Buick Park Ave NA