You need to clean the valve face where the washer seals against.
If the previous one has been leaking for a while the seat may be badly pitted and the only option then would be to replace the complete D.o.c.
I suspected you meant MT= empty=draw-off cock washer, but I wondered if you meant, are you sure you want to use a DOC washer mate? Or if MT meant something else. But fine, I get what you meant now, thank you.
Reply to Drain off cock washer in the Plumbing Jobs | The Job-board area at PlumbersForums.net