Setting the parking brake will have no affect on the left brake, as the parking brake lever only sets the right brake.
What kind of condition is the short rubber flex line in, that attaches to the wheel cylinder..?? And what kind of condition is/are the fitting(s) in where the rubber line connects to the metal line, on the master cylinder side..?? I'm wondering for some reason, when you activate the right brake using the pedal, and parking brake, if there is a flap, from deterioration on the inside of the rubber line pops up, and blocks fluid from returning. Theoretically, it maybe should do it when using the pedal alone, but then again using both may make a sudden rush of fluid to flip a small piece on the inside up, and block fluid from returning. I have heard of rubber lines on older cars with manual/not power brakes doing that.
That's why I was wondering what condition that connection is in, where it connects to the metal line. If you could crack the line there to relieve pressure, instead of the bleeder valve, if no, fluid comes out to release the brake, you've found the problem. If it does, you're back to square one, and could very well be the master cylinder. I just don't want you to twist the lines, and have to replace them. Trust me, you don't want to buy a preformed line from Case. I had to replace the right brake lines on mine, (there are 2). For the price of those 2 lines, I bought a double flare tool from Eastwood. and 2 rolls of Ni-Cop line, and a variety of fittings, to make them for less money. And still had enough line to make at least 4 more, plus the fittings.
If it would happen to be the rubber flex line that is the problem, you'll get it 10% cheaper through a Case/AG Ag. dealer, than a Case Construction dealer. I had to replace mine, and really lucked out and found an OEM one a guy had on ebay, for $20 less than dealer price. That little rubber line is $53 at Case Construction. $48 at an Ag, dealer, just checked prices. Higher on ebay, and WAY higher at Amazon. Broken Tractor has an aftermarket one for for $38.00.
https://www.brokentractor.com/p/case-brake-hose-from-slave-cylinder-to-steel-line-d66741-d66741/
If it would happen to be the master cylinder, through Case Construction, they are $356.50 ea. I just found an aftermarket one on ebay for $53.95 w.free shipping. Back when I did mine approx. 8 years ago, I got mine here
Remanufactured Transmissions, Torque Converters, Engines - Joseph Industries $48 each. They still work as they should, so no complaints here. Also got a water pump for it 3-4 years ago, for less than anyone else online. It too is holding up so far. They don't list too many prices, so you have to call and check. They are located approx. 120 miles from me, and if I order before 10:00 am, parts are here the next day. You may even find one for less somewhere else, if you use the part number to look for one. You may find that neither is the problem, but just remember if you do need a part, use the OEM part number to do your search.