When I towed the boat from Kansas City to Table Rock for the July 4th weekend, the brakes worked great all the way down. Several days later after pulling the boat out, I quickly discovered that my trailer brakes were nearly non-functional. I found the master cylinder was nearly empty and brake fluid was all over the safety cables, etc. I didn't have time during that trip to investigate and was able to get the boat into storage.
On the next trip, I filled up the master cylinder but didn't have time to bleed, etc. The brakes worked virtually as well as normal even without bleeding, but I had to top off the MC a couple of times during the week. At the end of the week I took the boat to the shop for winter maintenance and had them look into the fluid leak since they're a lot better equipped to do it than I am now that the boat is 3.5 hours from home.
One thing they found:

They weren't sure if that was the source of the leak and after looking at it closely I'm not either. That tiny crack at the very end of the fitting is probably more suspect than the broken rubber jacket. In any case, I had them replace the whole actuator and install my spare new lockout solenoid. They had NAPA make up a new brake hose and I had them make it 8" longer than the original, which was always too short to swing the tongue more than about 60-70 degrees. (I've always known that and never "forced" it any farther for the sake of the hose, but it's possible that a shop has tried to swing it too far sometime over the years, breaking the jacket.)
Anyway, don't forget you've got a rubber hose in there.