You may have a piece of debris not letting the valve blade fully seat. I had your symptom once - dumped the tank, cleaned out w/ lots of water and a wand down the toilet, backflushed w/ my Flush King, and that seemed to take care of the issue. I always now put a bucket under the pipe end when removing the cap just in case
Mike
__________________
Mike and Kim
Current TT: 2012 Eagle Super Lite 308RETS
Current Hitch: Equal-i-zer 1400/14K
Current TV: 2010 Chevy 2500HD, CC, LB, Duramax/Allison