Not saying I have the answer to your problem(s), but just some thoughts. I would start by replacing the hydraulic fluid and filter, along with cleaning the transmission hydraulic pickup screen (it is back by the PTO). I included a link to the manuals. I would go through the fluid and see, if any think is in there. If you live far from the dealership, I would get the o-rings for the plugs. URL to JDparts included. It may not coast because the brakes are hanging up. With the tractor off and in neutral (gear and range), I would block both of the front tires, jack up the back, and see if you can turn both wheels freely.
Where is the metallic clanking sound appear to be coming from ie transmission or brakes? Are the rims hot to the touch or does it seem things are warmer under your seat and feet? Will the tractor go in/out of Park?
I think (hopefully someone will agree or disagree with me) the tractor is going into relief and that is why you are losing the brakes. This is not good for the pump.
I would not try doing anything without getting the technical manual, the first URL gives you the manual number and can be purchased from JD or you could look on e-bay. If it is the brakes you need the manual, if it is the transmission you need the manual.
You can also search this web-site, to see if someone else has had similar problems.
Sorry I do not have an answer, but hopefully this will give you a place to start looking.
John Deere Ag - New Equipment: Technical Publications - Publications Search.
John Deere - Parts Catalog
