Spring has sprung and trailers that have slumbered through the winter are being dragged out of storage and dusted off for use. Check the air in the tires. Light up the lights. Yes, we're good to go for another season!
Experience has taught me that "spring cleaning" seldom includes the trailer's surge brake. I would set up at a brake check and start waving in anything pulling a utility or boat trailer for inspection. It did not take long before I found a trailer with serious problems or even no functioning brakes at all.
The first sign of trouble was the breakaway brake cable. This was often frayed, broken or missing. This is not optional equipment. If it isn't in good condition and long enough to attach to the tow vehicle the trailer must not carry a load that requires brakes.
Next I would hand my adjustable wrench to the driver and ask them to remove the cap on the master cylinder for me. The amount of force employed was a good indication of whether the brake fluid level was checked before this voyage began. Occasionally the driver destroyed the cap trying to remove it.
Brake fluid is a clear pale yellow liquid that must fill at least half of the reservoir. Ideally, the level should be about 3/8" or 9 mm below full. If it's rust coloured or not there at all, again, the trailer must not carry a load that requires brakes.
If the breakaway brake cable and brake fluid quality and amount looked serviceable, the final check was to ask the driver to apply the breakaway brake and try to roll ahead. Of course, the trailer should resist being moved.
Where the actuator roller is visible it should not travel more than an inch or 2.5 cm when the breakaway brake is applied. If it does, this is a sign that the brakes are out of adjustment.
One surge brake owner's manual that I read suggested a field test of the system before the trailer is connected to the tow vehicle. The trailer is parked on level ground and the safety chains are attached to each other in a loop under the hitch. A length of 2x4 lumber or other suitable prying tool is inserted into the chain loop and used to lever against the front of the coupler.
The coupler should move to the rear and apply the brake. This will prevent the trailer from rolling away from you as you continue to push.
Choosing to tow a load that requires trailer brakes when the brakes don't work is inviting disaster and police will likely treat it as such. The trailer could be towed or removed from the highway until the load is reduced or repairs are completed. Fines and inspection orders are also an almost certainty.
This trailer article reminds me of a court decision I read on your site, where some idiot near Ucluelet overloaded his poorly maintained boat trailer and as a consequence some poor woman out picking berries got killed when he lost all control of his rig.
Another aspect of boat trailers that people often don't think about is that repeated immersion can flush all the lubricant out of the wheel bearings, making them likely to overheat and jam (though maybe they have better sealed bearings these days and I'm out of date on this).
Weirdest thing I ever encountered was some years ago, backing a friend's Explorer into the water with a trailer on it, in order to float his boat. The wheel sensors for the ABS & Stability Control didn't like the immersion one bit and the whole dash lit up like a Christmas tree!
Me, I never get anywhere near these things on the highway. I've seen too many under-inflated tires, and drivers who didn't understand how to place the weight, oscillating down the highway.