If you have humming noise at the wheels you have power going to it. I check my brakes at every trip that way.
1. Your brakes are worn out and will not squeeze when energized or need adjustment.
2. Someone pumped grease in you drum thru the zerks and now your linings are greased and just slide.
3. You have a bad ground in the system and the voltage realy drops fast when you apply the brakes.
4. You might want to check one wheel for grease condition
On my new trailer the Hubs were greased improperly and I had grease stuck to my magnets and had to clean all 4 hub faces, I was able to save the linings.
Its a common problem, I just towed a TT yesterday and I noticed that 2 of the 4 wheels were draging everytime I touched my brakes very lightly, so I told the owner of the unit to have the brakes checked for improperly greased hubs before taking the trailer on a trip. But he told me I just bought the trailer, well I told him they didn't service it properly.