If yours is the typical rider lawn tractor similar to about most lawn tractors which can be purchased at Sears or Home Depot, etc, it can be a couple different things. I had the same problems on my free junker Craftsman lawn tractor, as well as an older MTD tractor my parent's used to make me to mow the lawn with, before I moved out and went to collage.
The brake/clutch pedal activates a mechanism under the tractor frame, normally in the area under the engine. That mechanism has a pulley which rotates on an axle when the brake/clutch is depressed. The pulley is the tensioner for the drive belt going to the transaxle. The axle which the tensioner pulley assembly rotates on normally rusts and binds up. I disassembled mine and cleaned all the rust off, and it worked fine. When I would press down my brake/clutch pedal, it wouldn't come back up. It would disengage the drive belt and lock the brakes simultaneously.
Another thing to look at is the brake disc on the rear transaxle. It normally has a threaded steel rod hooked to a lever, and the lever pivots when the brake/clutch pedal is pressed. As the lever pivots, normally there is usually a cam mechanism which squeezes the brake disc between two pieces of friction liner material. If the brake lever on the transaxle binds, it can keep pressure on the disc and the brakes drag or lock up.
The third thing to look at is that the steel rod which attaches to the brake lever normally has a spring between the a nut on the threaded part of the rod, and the lever on the transaxle. As the threaded rod pulls forward, the brake action is softened by a compressing spring. If this spring is too tight, the brakes will drag.
Sorry if my descriptions all seem very generic. They are that way because I've never worked on your type of lawn tractor before, so I have no actual experience with it. However, what I've described is pretty much how most lower cost lawn tractors are built, so from the generic description, it should give you an idea of things to look for. Hope it helps.
