Abstract: An apparatus for maintaining hydraulic pressure in a spring applied hydraulicly released brake actuator and activating a parking brake light upon stroking the actuator to apply the parking brake in the system, the apparatus including a housing, a compression spring axially aligned in the housing, a cap mounted on one end of the housing, a cylinder formed in the cap, a piston mounted in the cylinder, a spring cup operatively connecting the piston to the compression spring, the piston rod being operatively connected to the piston and to the brake, a tubular housing having a magnetic switch at each end axially aligned with the piston rod, and a cylindrical tube having a magnet mounted thereon aligned with the tubular housing for actuating one or the other of the magnetic switches, one of the switches being actuated on a drop of pressure in the system and the other switch being operatively connected to a parking brake light to signal a loss of pressure in the system and actuation of the parking brake.