I took apart the master cylinder on the boat trailer yesterday and cleaned it. It’s been sitting dry for 10 to 20 years. I cleaned the cylinder walls as best I could and reassembled it but it seems to leak a bit. Still, after adding fluid and bleeding the lines, the trailer brakes seem to work.

How would you know if the trailer brakes are working without doing something extreme?

I’m going to do a little research, thanks to the internet, and maybe find a rebuild kit for that master cylinder. Maybe it just needs new rubber parts.