Normally Open Free Backing Electric Solenoid Valve. Commonly found on boat trailers, the backup solenoid is required if using disc brakes with a surge coupler to prevent locking of the brakes while reversing. This Normally Open Reverse Solenoid operates on a 12 volt capacity and installs with two wires. One wire (either one) connects to tow vehicle reverse lights. One wire (either one) connects to ground. This valve is rated to 800 p.s.i. The solenoid is activated when the tow vehicle is shifted into reverse, powered by the reverse lights. The solenoid then closes the brake fluid port and allows the trailer to back up without locking the trailer brakes by preventing brake fluid from reaching the brakes on the trailer. Male threads (1/8 MTP male pipe threads) screw into back of master cylinder. Female threads (3/16 inverted flare) connect to standard brake line.