Bleeding is the service people ask for most often and need least often. Before booking it, a few other things produce the same symptom and are worth ruling out.
When it really is air
- The lever feels spongy and the bite point moves from ride to ride.
- Pumping the lever a few times firms it up — trapped air compressing.
- The bite point is better with the bike upright and worse after it has been upside down.
When it is something else
- The lever is firm but stopping power is poor — most likely contaminated or worn pads.
- Squealing or juddering under braking — a warped or glazed rotor.
- The brake drags after you release it — a sticking piston, which needs a caliper service, not a bleed.
As a rule of thumb: once a year is enough for most riders, and systems running DOT fluid need it more often than mineral oil ones, because DOT absorbs moisture from the air.