That's a surge brake coupler I'm guessing.
There may be bushings or slides that can be replaced, you'll have to find the brand and model of the coupler to know. I doubt the issue is hydraulic, likely just wear and tear in the slide mechanism.
I personally hate the stupid things, way over complicated, super expensive, and they never seem to work right for long. Electric brakes are way better but that's a lot of work and expense to switch over.