i agree with haysse, it sound like you need a reaming tool, which you can get from mitre 10 or micos, or mastertrade. You insert that into your tap were the washer sits and simply turn it around a bit, and little peices of brass will come off making the tap seat smooth, rinse those bits of brass away and try using a black washer instead of the typical red washer for the hot, it much softer and more plyable.
aj
July 2012