During a pre-trip inspection of a commercial vehicle equipped with hydraulic brakes, which of the following checks is a required step to ensure the system is not leaking?

1. Pumping the brake pedal three times and holding it down firmly for five seconds.
2. Checking that the air compressor cuts out between 120-140 psi.
3. Draining the air tanks of any moisture and oil.
4. Ensuring the slack adjusters have no more than one inch of free play when pulled by hand.