Inspect linkage for binding, lube all moving parts with oil squirt gun making sure not to get oil on belts, grease all fittings with grease gun. Push hard on brake pedal while disengaging brake lever by hand. I have had same problem before and found linkage binding, after lubing worked pedal and hand engagement lever several times and resolved problem. After driving tractor around try it several more times. Let us know how it works out.
