"Oh yeah, I know the trailer brakes are working, I could feel them grabbing on the hill!" To me, this was almost as reliable as "The cheque's in the mail!" when it came to testing trailers with surge brakes at the roadside. This driver used the wrench I offered him to show me that there was no brake fluid in the master cylinder of the surge brake. Surprise!
As far as the brakes were concerned, this driver should have hooked up the trailer, checked the brake fluid level and condition and then conducted a tug test to ensure that the brakes were working before he left the driveway. It's simple, as aside from the wrench, all you have to do is set the breakaway brake and try to pull ahead. If the wheels don't lock, that's the end of the trip until they are repaired.
Brake fluid does not evaporate. If it is low, it is because there is a leak. It is definitely not good practice to top up the master cylinder and carry on.
Brake fluid is also hygroscopic, meaning that it will absorb water from the air. If the seal on the master cylinder is broken, the water that will accumulate in the fluid will rust the inside of the braking system rendering it useless. Keep seals in good shape and follow your trailer manufacturer's recommendations for replacing the brake fluid.