What causes this many times is the weld at the rear of the top plate has pulled through the deck. When tension is put on the belt, the plate pulls up forcing the arms into the pulleys. Reinforce and reweld or replace the deck is your repair choice.

What is the purpose os the arm brake? One of mine is broke at the pad. There is tension on the brake so that the pad is pushing into the belt. Does the brake ever not touch the belt? If so, how? Think I am missing a spring or something. I have searched this forum and the Internet for a picture. Can't seem to find one.

look here: sears parts link

Here is a link that might be useful:

how do i put the arm brake and spring on