Why does the AAMVA manual instruct drivers to open each trailer air tank's drain valve daily, even if the tank has an automatic drain?

Question

Accepted Answer

To confirm by hand that water and oil aren't building up in the tank, since buildup can keep the brakes from working correctly

Answer

Because automatic drains are illegal on trailers built after 1998

Answer

To relieve excess air pressure that the governor failed to cut off

Answer

To test whether the ABS malfunction lamp is working