One of the purposes of a proportioning valve is to restrict pressure to the front brakes to avoid front wheel lockup on slippery surfaces like mud or snow. The rear brakes only are supposed to apply with light pedal pressure so the vehicle can still be steered- - - -locked-up front wheels tend to plow ahead in whatever direction the suspension is pointed under slippery conditions. It's possible your valve is clogged, or the opening pressure is way too high to the front brakes for some reason. Try bypassing the proportioning valve temporarily and see if the brake performance improves.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!