Visually inspect the stanchions of your shock. Some have a rubber O-ring, or there may be a faint line of oil showing you the total extent of your travel. If you’re not using the full travel, you’re probably running too much air in your shock.
If you don’t have an O-ring fitted, you can use a plastic zip tie fastened around the stanchion. Go for a ride, and at the end the zip tie’s position will tell the story. Don’t leave the zip tie on though as it can damage the surface finish.
Remove the screw-on dust cap from the shock, exposing the threaded Schrader valve. Screw the shock pump chuck onto this. The dial on the pump will display the shock’s current pressure; make a note of this.
Add pressure with slow, deliberate strokes. Use the pressure release button if you need to drain some air at any point. When finished, make a note of the new pressure before removing the pump.
Setting shock sag (negative travel) is vital if you’re going to get the best performance, and can vary from a quarter to a third of total travel. Aim for this and adjust as necessary. For a softer setting, set sag at around a third.
To check sag, have someone else hold the bike while you climb carefully on. When the shock has been compressed, climb off again: the O-ring or zip tie should show how much sag you have. Adjust to the quarter or third of travel point.
When you’re satisfied that the shock is set up with correct sag levels you need to record your pressures, so you can reset the shock to your own preferences after any servicing. Some riders glue them to the pump shaft.
Air loss in modern suspension units is rare but it can happen, so keep your pump in good condition and take it with you on rides: every now and then you might find that you or someone you’re riding with needs a breath of air.