Purge valve keep going off

[3406E]

Truck 05 9900 eagle isx. Purge valve keeps going off. Builds to its normal pressure 120 and then starts to purge.Sometimes every two minutes, sometimes 6 minutes. Changed to a brand new dealer bendix dryer and still have to drain about a half cup of water in mornings. I’ve been checking for leeks but cannot find any. The purge valve doesn’t go off often when idling only when driving. To add sometimes I go to drain the air tanks and there is no water coming out as it should, but after few days it’s starts again.
Any suggestions?