A percussion device for a hydraulic rock drilling machine (1) including, inside a cylinder (4), a to and fro moveable impact piston (3), which is provided with a land portion (12) for cooperation with a braking recess (11) in the cylinder (4) for the establishment of a braking chamber in advanced positions of the impact piston, wherein a throttle slit (17) is arranged to be established between the land portion (12) and the braking recess (11). A pressure channel (14), being connected to a pressure medium source, is arranged to debouch in the cylinder (4) between the piston guiding device (6) and a position of the land portion (12) in a most advanced position of the impact piston (3). The invention also concerns a method and a hydraulic rock drilling machine.