There is disclosed an improved sliding exhaust brake system installed on a large vehicle. The brake system comprises a housing, an exhaust gas passage, a main valve member taking the form of a flat plate, an exhaust pressure-adjusting hole formed in the main valve member, a block member fixed to the front end of a piston rod, and a piston valve mechanism for opening and closing the exhaust pressure-adjusting hole. When the passage is closed by the main valve member, it is pushed in one direction by the front end of the piston rod. When the passage is opened, the block member engages the main valve member to move it in the opposite direction. The piston valve mechanism is biased forward by a spring.