Common point of confusion is there are TWO types of TAs.
Starting with the Super MTA, and up through the 560, the TA was MECHANICAL. If a mechanical TA goes out on the low side, you can run the tractor virtually forever on the high side with no problems.
When the 806 came out, the larger tractors came with a HYDRAULIC TA. Anything made between 1963 and 1985 with a 7, 8, 9, 10, 12, 14, 15, 30, 32, 33, 34, 35, 36 or 37 as the first "digit" in the model number has a HYDRAULIC TA.
The hydraulic TA is a totally different animal. When the low side goes out, the high side is usually not far behind.
IMHO, never buy a tractor with a hydraulic TA that has TA problems unless you plan on replacing the TA immediately.