Brake wont release on its own, power problem?

My question is two fold and possibly connected. The brake will not release on its own i must pull it back with my foot. I have replaced springs and made sure assembly is moving freely. Mower also struggles sometimes on hills but engine does not bog out. It seems the belt is either slipping on engine pulley or just not turning. Mower deck always runs fine and carb has recently been cleaned. Could the brake be the whole problem or is it likely the pulley. THe mower is an ayp model y1844. The engine is a kohler mv18