What action actuates a dead seat brake and causes forklift to brake?

The release or reduction in the hydraulic oil pressure that holds the fork lift in a "raised" position. When these actions occur, the weight of the forks and load cause the dead seat valve located in the lift cylinder housing to unseat from the cylinder ports, allowing the hydraulic oil to flow back to the sump. This causes the lift cylinder piston to move down and the forks to lower, which brakes the forklift.