It is the dampening shock in the coupler, and your brakes could have air in them also. Which if they have air, you have an issue. Trailer brakes are not like a car, they are sitting and much like an old farm truck that sits, they go south just from moisture absorption and heat. The heat in the lines allows fluid to form in humidity and then gassing, and then air, and then you have no brakes. They just dont usually get a lot of miles on them and it is like any product that sits some. Four wheelers, or other hydraulic slave systems. Try checking the lines for air, have them bled, and then check the shock dampener if all else fails.
