If your trailer has Reliable manufactured components there is a strong possibility that your calipers may be hanging. This was a known problem with the calipers supplied by Reliable and where part of a warranty replacement program.
I had this issue with my 03 MC trailer and was able to get a replacement set of Kodiak calipers from Reliable before they went out of business. Most everything Reliable sold was junk in one way or another. I still have their actuator on my 07 trailer and am doing what I can to keep it alive as long as possible. There are several threads about replacing the actuator with a UFP retrofit kit. Sooner or later I fear I too will be looking at this replacement and expense.