The 400 has a planetary pto that is easily adjustable from the outside, right on top under the little cover. Some times the drive band is so worn it will not hold and you can pull the back off pto and switch the two bands as the brake band seldom wears unless totally missadjusted.
Follow the book procedure if you have a book. Other wise, align the lock half way between on and off. Tighten both bands down snug. Back them each off 1 1/2 turns . Now, with a short punch, 5 or 6 inch, you should be able to turn the shaft with some effort with latch half way between.
Due to worn linkages some may require some additional adjusting but can't tell that from here.