PTO Stopped PTOing

My farmall 300's PTO did not engage,it was just used 2 weeks ago. The oil was white (water) so I changed it - but no go. I hope I don't have to tear it down.

If the oil was so bad that it is the cause of your PTO not working, just changing it will not fix the problem. This is because the lack of lubrication would have torn up the insides of the PTO.

The oil was not your problem. It was a good idea to change it but something else is wrong with your PTO.

Does the lever "feel right" when you try to engage the PTO? If it feels different, look at it and see if there's something wrong with the pivot point or the linkage to the PTO.

The only other (unlikely) cause of the problem is if the seasonal disconnect under the belly of the tractor came undone. Again, it is very unlikely that this happened, and not all tractors have seasonal disconnects.

Did you ever hear any strange noises, grinding, clanging, clanking, crashing, smashing while running the tractor over the last two weeks?

Do you have the IPTO or the transmission driven type that looks like what you would see on an H? If it is the IPTO I do not have much experience with them but I would start by checking the tension on the adjustment screw, the drive band screw is the one closer to the front of the tractor underneath the peanut shaped cap. If you have the transmission driven PTO it is much simpler and much less to go wrong.
Zach

The linkage on well used IPTOs on the Farmalls (and utilities) become dysfunctional after years of use.
Two suggestions:
Make very sure there is no lost motion in the hand lever pivot and rod mechanism. Be certain the hand lever moves the PTO lever from the middle both ways.
Adjust it like this, after there is no play in links.
Move the lever exactly half way between the locked on position and locked off position (little flipper catch on lever.
Take off the little dome metal cover from the adjust screws.
Loosen the jamb nut, and tighten the front (away from you) screw until it is just a bit tighter than the point at which it begins to feel tighter as it is screwed in (maybe a eighth turn) then back it off until the PTO shaft will turn with least drag (about 3/4 turn out) and tighten the lock nut while holding the adjuster.
Do the same for the closer adjuster.
If the screws go into the jamb nuts and get flush with the top surface of the nut, the bands are probably worn out, and may be operational for a while, but plan on a rebuild.
Now (tractor off) push the the lever down toward the brake footplate. As it goes down, it should get harder to push when it is about 4" (or so) away from fully OFF. and be noticeably harder to push for the last inch, but not real hard. (this is the brake band tightening on the drum in the PTO unit.
Then pull it up and do the same feel.
The ON position should feel the same way.
The shaft should turn pretty free when the lever is half way, and not at all when in either ON, or OFF. (tractor shut off)
If it slips now, it needs a rebuild. JimN