You’ll find the tap washer can be in all kinds of condition it may even look ok which means the seat of the tap must be damaged, sometimes the rubber washer will get stuck in side the tap when you pull it out so use your long nose pliers to remove the rubber washer ,
After removing the washer place you finger down inside the tap and gently twist your finger to feel what condition the seat is in.
If you’re unsure then attach the reseating tool to your drill firmly. Screw your reseating tool on to the tap like you would be reinstalling you tap , screw the reseating tool down clock wise by hand not too tight then hold the drill down firmly and turn the drill on for approximately 3 to 4 seconds.
Tip: Before turning the drill off, lift the drill up so the reseating blade doesn’t stop hard on the seat of the tap and leave a mark. Gently push down and turn drill on, lift the drill up then turn the drill off ,remove the re seater and have a look down the tap and you will see the brass seat will be shining and if there is a little mark that has worn through the seat it will show up very clearly it will be a dark line , you will have to attach the re seater again and repeat the process , repeat until the mark has gone, there will be a build up of brass shavings in the tap so remember to leave tap on then you turn mains water back on to flush out the shavings before you turn the tap off so it’s nice and clean inside the tap and you haven’t squashed down brass shavings onto you nice clean seat , taps inside the house usually have a small filter at the end of the spout make shore you remove the filter before flushing brass filing out otherwise they will clog up the filter and you will have very poor water flow.