Faulty brake valve on towing vehicle. No trailer load. Brakes need adjusting, lubricating or relining. Grease on brake lining - reline brakes. Brake shoe return spring broken.
Brake drum out of round.
Leaking brake chamber or actuator diaphragm. Restricted tubing or hose. Relay emergency valve or drain valve leaking. Leaking tubing or hose. Hose uncoupled or leaking hose coupling.