Sounds like a master cylinder problem if there is nothing obvious showing, usually the seals wear and pick up minute quantities of air over a period, these all add up.

Other things could be excess travel on the slave cylinders, this would allow small quantities of air into the system also, as would faulty or wearing seals.

Fourtrak brakes are spongy feeling by nature, this is actually a design feature of the vehicle, yes a design feature, not a cock up as many manufacturers make and try to pass off as a feature or trait.

Due to the fact that they were designed to tow heavy loads, this slight sponginess prevents sudden braking from happening, the trailer brakes then operate properly. Basically a damper in the vehicles brakes, effectively; modern car brakes snap on quickly, this does not allow time for the trailer brakes to apply, this them pushes the towing vehicle out of control.

Would suggest you find another owner who will let you drive their vehicle for a short distance, you can then compare your vehicle against theirs.