For hydraulic systems to work efficiently they must rely on an in-compressible fluid as a means of transferring forces.
In hydraulic braking systems the input forces you create by operating the brake lever travel via the brake fluid to operate the caliper pistons, which in turn, move the brake pads to contact the rotor. This is only possible because the brake fluid is in-compressible.
Think of brake fluid then as a means of communication. The brake fluid transmits the message given at the master cylinder (lever) to the slave cylinder (caliper) with minimal loss in quality.
Air in the system causes a breakdown in this communication. Input forces created by operating the brake lever can no longer be transmitted effectively as these forces are wasted as the air inside the system is compressed.
So how do you know when air is present?
There is one classic sign of air inside any hydraulic mountain bike brake and that is excess lever travel - sometimes referred to as a 'spongy' or 'loose' brake lever.
If the lever has to be pulled a good distance before you feel the pads engage (or bite) the brake rotor then there is a good chance it is down to air inside the brake fluid. Take a look at the following diagram:
On the left we can see that it is possible to pull the lever back to the handlebar. Here the brake system has too much air inside resulting in little to no stopping power. This brake needs to be bled before it can be used.
The middle lever still has to travel a good distance before the bite point is reached and the brakes start to work. This suggests that air is present in the system but not enough to render the brake completely useless. This brake should be bled to create a firmer feeling brake lever.
The example on the right is what we consider to be a healthy brake lever feel. Firm, sharp, confidence inspiring. This is how your brake lever will feel after a successful bleed.