A compression release engine brake assembly adapted for use with an internal combustion engine, and an exhaust valve (29) operable to open in a normal exhaust lift event (FIG. 9) and in a brake lift event (FIG. 15). The engine includes an exhaust valve actuating mechanism (23,31,37) for imparting reciprocal movement to said exhaust valve (29) in response to rotation of a cam shaft (11) including a cam profile (13), a normal lift portion (17) and a brake lift portion (19). The assembly includes a lost motion device (75) moveable between a normal lost motion condition (FIG. 4) and an actuated condition (FIG. 14) in response to movement of an input member (91). An energy storage spring (117) is operable, after being compressed to an energy storage condition (FIG. 12) to be able to bias the input member (91) toward a second position, effecting the actuated condition of the lost motion device (75). A latching mechanism (99,103-113,129,131) is operable to first displace the energy storage spring (117) to a compressed condition, and then release it just before the cam follower (21) traverses the brake lift portion (19), in response to the pivotal movement of the rocker arm assembly (23) which comprises part of the exhaust valve actuating mechanism.