Old post, but in case it's of any value - Lou's description is correct and in the case of the OP's trailer, the solenoid is of the 2nd type he described. When the solenoid activates, all it does is open a valve (internal to the solenoid) allowing brake fluid to return to the master cylinder rather than pressurize the brake lines.
And these solenoids aren't terribly reliable. I'm on my 3rd; the original one failed before the end of the first season. The 2nd lasted 8 years and still wasn't completely out, but was beginning to go.
I posted some pics on this thread long ago, in case it's of any help with the LP70 or LP7000 actuator:
viewtopic.php?f=12&t=6235&p=60390#p60390