Bleeding trailer brakes is fundamentally the same as bleeding brakes on a tow vehicle. You must first detach the trailer from the tow vehicle and block the trailer, making sure the master cylinder is full of fluid at all times. Open the bleeder screw that is furthest away from the actuator. Apply force to the actuator slide, hold it, and then tighten the bleeder screw. After this, release the actuator slide. You should repeat as necessary until all air is cleared. Continue to the next furthest bleeder screw and repeat until the entire trailer has been bled. Be sure to check the master cylinder fluid level frequently during the operation. Also, if a new actuator is installed, it should be bench bled, or pre-bled, before it is attached to the hydraulic line at the rear.