The gasket in your group head, made of rubber or silicone, is what actually makes the seal against your portafilter. In a home setting, your gasket will probably last about 6 months to a year or maybe even more, depending on how much you use your machine. With age and use, the gasket will get harder and more brittle, making it more difficult to make a seal.
The biggest sign that it’s beginning to fail is dripping or leaking around the edge of the portafilter as you pull a shot. Another indication would be if you start having to pull the handle over further and further to the side in order to seal the portafilter to the group head.
Removing the gasket usually requires a sharp tool known as a gasket pick, or you can also use an awl (handy if you decide to take up making belts). After poking the awl into the gasket, you’ll either be able to pull the whole thing out in one piece, or if it’s older and more brittle, you’ll have to pick out the pieces bit by bit. A pick or awl can get into those tiny spaces and pry everything out.
Use a brush to make sure the surfaces are clean before placing in the new gasket. Bits from the old gasket left behind can affect the seal.
The gasket will have a smooth side and a side with printing or indentations on it- when installed, the smooth side is the one facing the portafilter.
THE DISPERSION/SHOWER SCREEN
The dispersion screen is where water flows out over the coffee bed. After every shot, run some water through the group head to help clear out any coffee grounds that may be stuck to the screen. Every couple of days you can take a brush and scrub the screen and the gasket to get any loose grounds.
Once every week or two it’s a good idea to take off the dispersion screen and clean off any coffee build-up on the inside of the screen.
The screen is held on to the group head in a few different ways- depending on the type of machine you have, there will be a flathead or hex screw holding it in place, and you will need a stubby screwdriver or hex key to take it off.
Some machines also have a dispersion plate or block that will come out at the same time as the screen- that block can accumulate oil, too.
If your screen and block don’t look too bad, a little warm water and a soft cloth should clean everything off. If you have a sticky layer of coffee oil and fines (super small coffee grounds) built up, soak everything (screen, screw, dispersion block) in hot water with cleaner to help loosen everything before you scrub it down.
NOTE: There are some dispersion screens that are treated with a coating that will come off with detergents, so keep that in mind before throwing everything in a soapy bath.
When you’ve cleaned everything, reassemble and run water through the group head to do a final rinse, and you're ready to go!