PURPOSE: To protect a brake cylinder from very high pressure by fitting a means of clogging a relief flow passage if working pressure exceed a prescribed pressure.

CONSTITUTION: The spring force of a compression spring 19 is designed so that fluid may flow freely into the path 24 of a relief flow passage from a flow passage 30 by raising a poppet member 20 so as to leave a valve seat 22. Only when the working pressure in the flow passage 32, or the working pressure in a wide valve hole part 16 increases and exceeds a prescribed value, a valve spool 12 moves against the force of a spring 19, so that the poppet member 20 comes into contact with the valve seat 22. As a result, a relief flow passage is closed, and hydraulic fluid will not escape from the chamber of a brake controller 100.