What would be the difference if air were used in brakes rather than liquid brake fluid?

The transfer of force by the application of liquids is
known as hydraulics, while the same done using gases is known as
pneumatics.

There are some advantages of using liquids
rather than using gases. In vehicular brakes these advantages make it easier to use
liquids in the braking system and they are given a higher preference than
air.

The advantages of using liquids include the fact that
liquids are almost incompressible and this prevents any absorption of the energy that
has to be transferred. The fact that liquids are not compressed also increases the force
that can be transferred. When gases are used as a medium to transfer forces, they retain
the pressure that has been applied and this results in the piston that is used to apply
the force springing back when the pressure is released. To solve this problem special
bleed off valves are required. The use of liquids does not lead to this problem as a
small movement of the load releases the pressure that is
stored.

Using air instead of a liquid in brakes would make
their use quite difficult as the initial force to be applied would be higher. When the
pressure would be released from the brake pad it would spring back suddenly and there
would also be a necessity to use a separate bleed-off valve to eliminate the pressure
that is stored in the compressed air once braking is not
required.