When the parking brake is applied, the passage for charge oil
(15) is blocked at the brake interlock valve. The springs push
on brake piston (14). This compresses brake plates (10) to
brake discs (11). At this point, coupling (13) is locked to
stationary housing (12) and the drum will not turn.
When the parking brake is released, the charge oil is routed to
the passage for charge (15) by the brake interlock valve. The
charge oil enters the brake piston cavity and the charge oil acts
against brake piston (14). The charge oil pressure overcomes
the force of the springs. This moves the brake piston away
from brake plates (10) and brake discs (11). At this point, the
drum is free to turn.