A brake applying mechanism having a chamber with inlet and outlet ports, a piston slidably arranged in the chamber and a valve to be controlled by an operator for admitting fluid under pressure from the inlet to thereby act on the piston in such a manner as to cause it to advance in the chamber. The piston is returned from the position to which it advanced under fluid pressure by a return mechanism normally opposing advance of the piston. Upon failure of inlet fluid pressure the piston can be advanced manually by the operator unopposed by the return mechanism which is now rendered ineffective.