Hayden, The A/C problem has evolved (mutated?) over time. I was part of the problem, not cleaning the screens often enough. There are two in front of the radiator. One for the radiator and one for the A/C condensor. I had not been aware of the one in front of the condensor (my fault, it is in the book). There is an air filter at the top rear of the cab that needs to be kept clear. I don't know if my A/C is fixed or not as it is hard to put a heavy load on it when ambient temps are so cool. It might be.
The A/C belt was stretched beyond where it could be tightened and may have been defective. At first the A/C worked good untill you had been running the tractor a few hours and performance would drop. It would squeal at idle in a random way but quit at mid range or higher RPM. Later it got to where it failed sooner and sooner and would intermittently squeal at working RPM. As the tractor was under waranty and the tech had been reticent to "overtighten" the belt for fear of doing bearing damage, I DID NOT tighten it any myself. I did clean the pulley and belt with brake cleaner and apply belt dressing after clearing it with the tech. Seemed to help for a few days.
In retrospect as I think back over the progress of the problem, it is consistent with continuing deterioration of the belt (stretching). I think the cab air gets heated just a bit when the temp control lever is all the way to the cool end and the A/C is on FAN only. The tech pulled the top off the cab and checked the controls and said that the heater core water supply valve was going full closed and was adjusted correctly. If the heat never turned completely off then that would contribute to apparent reduced A/C function. I'm not so sure it turns off completely and will put a thermometer to it and see what the temp of the air is coming out of the vents with A/C off and fan only. It shouldn't be too much higher than ambient outdoor temp. If it is then maybe the valve is defective or just a bad design.
If you like the tractor, don't let lack of liquid fill disuade you. I'm getting along with the so so governor. The backhoe deal was a dissapointment. There are good aftermarket backhoe solutions WITH SUBSTANTIAL SUBFRAME that are acceptable. I would avoid 3PH backhoes. I may opt for a separate backhoe like a Cad Digger or whatever the spelling is, the DIY self contained backhoe or get a name brand subframe mounted unit.
A while back kubota dealer said there was a backhoe coming out with extended subframe that would clear the L4610HSTC's cab.
Maybe it was only a rumor but I was told that the 50-- series was a less frills lower cost (maybe not so robust) line. There is an awful lot which is right and good with my tractor and I am not the easiest customer to please. If my A/C IS fixed I'll be OK with it. The cab is a regular "green house" and when other folks are first bundling up to ride a tractor in the cold, I'm runing my A/C to avoid overheating.
The cab and heater are great when it is cold, nasty, and mixed snow/rain is falliing. Neighbors are changing clothes twice a day to keep from freezing while feeding cattle and I'm toast with a smile on it. Now that I have a hay spear for the FEL (pallet forks are dangerous round bale handlers!) it will be a BIG smile.
With nearly 300 hours of total tractor time on this, my first tractor, I can say that I'm glad I got what I got. It is a great fit for my needs and although I would have liked to be able to fit a Kubota backhoe, I think having a cab is worth the small loss. I can fit a subframe mounted backhoe that is sized for my size tractor if I just get off my duff and do it. I decided I didn't want liquid fill even if I could use it so that isn't an issue anymore. Tire shops around here won't take a tire with fill in it as it is considered hazardous waste. I may opt for foam fill and never have another flat.
Hayden, I didn't forget your temp where it fails question but it wasn't that simple since the problem was mutating as it went. If it was a belt as we now suspect, then there shouldn't be a problem. Before it got so bad I was comfortable with outside air temps over 100 F and high humidity (over 90%RH). When working anywhere near normal, the A/C made inside temps and humidity much better than outside.
I'll report any new developments on the A/C.
Patrick