Odds are you have an early unit with the brake pad. It was marginal at best when everything was working the way it was supposed to be.
In my experience, the biggest reason those early PTO units did not stop was because of warped clutch plates. In other words, you can put in as many new brake pads as you want, but it will still rotate until you replace the warped clutch plates.
If the clutch plates are indeed warped, then you need to be figuring out why they warped. This is usually because of a low pressure issue somewhere.