3/8 valve leaking?

every since a cold nite my dump valve wont close the whole way. it will shut completely when u fill the bag entirely up and im guessin b/c it has plenty of backpressure to shut then. but when the bag is half filled like i used to use it, it constantly leaks. i took the valve apart and cleaned it but it still leaks. its comin strait through the dump valve. anybody have any suggestions...its a 3/8 smc so if u know how to clean it or been thru the same let me know and sorry bout the paragraph