You can either remove the seal and clean it, which sometimes works, or replace it DIY for about $9.00. A part of the problem is the build up on the valve which should also be cleaned,or replaced.
Being a full timer I carry an extra seal and it takes me less than an hour to replace.
I also have a very stiff toothbrush that I have heated and bent to reach inside and clean the seal on the underside.
Harold and Linda
2009 CT Coachworks siena 35V
W-22 Workhorse 8.1L
Explorer Sport toad