Why won't my Murray riding mower stop after the drive belt was changed?

I believe it has to do with the main drive belt. (The brake on these types of mowers are almost useless.) If the main drive belt is to short it engages the slip pulley that drives the transmission. I replaced an original belt that was bad with a replacement that was about an inch shorter and my mower would not stop. Replaced the belt again with the exact size required and problem solved ... BUT ...About a month later the same thing happened. The mower would not stop when i released the drive pedal and i ruined my transmission "snapping it out of gear to stop the mower. Now i suspect it may be a worn "slip pulley" causing the main drive belt to ride closer to the spindle on this particular pulley, this , I believe , will cause the pulley to turn faster thus engaging the transmission. This pulley is similar to an adjustable type but it "slips" . On my Troy Bilt it is located in the rear right over the transmission. Hope this helps someone because i have spent hours trying to solve this problem.