dump valve need replaced?

i have a 91 isuzu pickup on airbags and whenever i park it i let it sit on the frame. Well after about 4 hours or so later the bags in the rear are filled with air. I cant figure out why its doing this.  

sounds like a leaky diaphragm  

what's the diaphragm?  

and how do i fix this problem?  

you need to clean the fill valves. What type of valves do you have? burnzya is correct, if your walves have a diaphragm they need to be cleaned basically you need to take the valve apart and there will be a spring and a rubber piece, the rubber is the diaphragm. clean it all with rubbing alcohol. This link may help but if it dosent let me know what type of valves you have and I will try to help

smc valve cleaning  
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World Wide Air Suspension Store
TheCustomConnectionOnline.com
270-860-1103

yeah my truck is doing this right now too. I aired down the same corner like 10 times driving home, after a perfectly fine week of driving. A hammer to the valve didn't solve it for very long   Was tempted to just turn down tank pressure and let it do whatever. So annoying.  

thanks!  

which valve would need to be changed? is this bad for my bags if i dont change them right away?  

if you're putting air in the bag, that's your lift valve... try using it a lot...play with it a lot. sometimes this stuff is moody, and it'll do it one day and not the next.
When you park it, you might run less tank pressure (that's what I did at least). That way you can't put that much air into the bag. Shouldn't be a problem either way. Just annoying.  

is the line to your rear t'd? 1 lift and 1 dump for the rear? (Compared to separate lifts/dumps per corner)