I have. x320 and know what your talking about, mine has done the same thing. Using your hand on the foot pedal take the pressure off it and push down on the lock with your other hand. It can be a pain to unlock sometimes, but mine has done this plenty.
I think that model has a helper springs To lift the deck. Jiggle the pedal with your left hand and try to push the lock lever down with your right hand. Once the lock lever drops you should be able to move (with your hand) the brake pedal to the rear position. You will have to overcome the spring tension that helps your leg lift the deck (that is not on).