Adjust brakes on sears riding 15hp mower?

You will need a 1/2" wrench. If you lay on the ground behind the mower and look underneath it on the right side of the rearend toward the front is an arm that comes from the front that attaches to a lever on the rearend. Where this lever pivots is the adjustment for the brake. You should be able to slowly tighten that nut until the brake drags when you push the mower back and forward. Once it drags just back it off a 1/4 turn and you should be OK. This should only take 5 minutes or less. The problem you may have is that after these brakes haven't been used in a long time they tend to freeze up. Once you adjust it and the brakes apply the first time you might have a hard time getting them to release. Many times I have had to take the brake system off of the rearend to free up the pistons in the housing. So, be aware that this might happen and allow extra time.