PURPOSE: To perform each of actuating and sealing operations so positively in delaying the suction and exhaust timing of a sealing medium to the supply of a working medium into a piston rod by selecting a three-way valve on one side to an exhaust brake operating position and then the valve on the other to a sealing medium supplying position, respectively.

CONSTITUTION: When an exhaust brake operating switch 31 is turned on, a first electromagnetic three-way selector valve 13 operates and thereby pressure in a working air chamber 9 of a spring return type pneumatic cylinder 1 goes up. Accordingly, a piston rod 2 moves to the closed side of an exhaust pipe 3, and when a slide valve 4 is engaged with an engaging part 26 of the exhaust pipe 3, internal pressure at the upstream side of the exhaust pipe 3 goes up, thus an engine receives a coercive load braking. At this time, a second electromagnetic three-way selector valve 25 is turned off after the elapse of a time set by a timer 32. Therefore high pressure gas is in no case taken into the operating air chamber 9 and a gas port both at the same time, thus the piston rod 2 is normally operated. In succession, a sealing medium is fed to a gas sealing part 21 from an air tank 11.