You only use the torque converter lock-up button when you want to keep a constant road speed or using PTO driven equipment. It basically turns your torque converter in to a clutched tractor. I’ve had my 545D for a couple years and it’s my go to. Mine is 10years newer than yours with an electric fuel pump and turbo but its basically the same machine from 1965. Its 2wd but pulls my 12ft. JD transport disc, a full HD 7ft. box and my 7ft. Woods 1000lb back blade.
If you’re over heating the trans, you’re in the wrong gear or its time to pull the radiator and see whats going on. I still have a lot to square away on mine cosmetically but its rock solid mechanically. I took a chance and bought it from an auction in Detroit. I picked up an OEM loader and wheel weights from Oklahoma last year for $1000. Hopefully, I’ll have the loader installed sometime in the next few months. A DIY QA, grapple and some forks are scheduled projects this fall/winter.
I dug out these stumps that were next to the house last week. The oak is 30” across and the pine was bigger. My Ford 4500BH loader couldn’t lift the oak or pull it out of the hole. The 545D pulled it out of the hole and across the field a 1/4 mile in 2nd low. There's about 1200lbs of wheel weights and 700lbs on the front end to keep it down.