I went through brakes nightmare couple years ago.
A definite you report is loosing fluid. So you have a leak somewhere. In my experience the leak wasn't obvious. Look for telltale gummy sticky drying 5606. You may not actually see a red drip. It is a high pressure system and doesn't take much a leak or ooze to feel soft. Unless it is obvious dripping you're going to have to count on feeling that tacky dry 5606 holding dirt to find the source. Start off with everything clean. A slight ooze you won't find if sticky 5606 has already attracted alot of dust. Moving parts highest risk for a leak.
Check the swivel. There were just recent posts on how to repair this. Mine were part of the leaking with fair amount gummy in the region. Had them redone and pressure tested.
I thought I had it resolved with that. But no. Long story short, my calipers were leaking. I disbelieved them as a source because they had been "rebuilt" which involves replacement of the o-ring at the start of the problem. So before I got back to them I went through everything else due to softness and fluid loss. I rebuilt the parking brake valve (easy, just some o-rings), replaced multiple lines (don't forget there's a flexible line in the sidewall by rear passenger seats that won't last forever), redid my master cylinders and the lines from cylinders over to parking brake valve. With persistent softness and fluid loss I was stumped until one day I took the calipers off for the umpteenth time and noticed gummy around them. Viola, the leak. And it was only ever so small with oozing around the o-ring, cylinder bore and piston. The cause was corrosion "micropitting" of the cylinder bore. I have attached photo. I thought I could hone it out but the metal is soft and to get deep enough to get the pits out "wallowed-out" the caliper. I ended up having to buy new (it's own nightmare of availability, superceeded parts numbers, supply chain issues and limited production runs and waits).
Link to the micropitting picture:
http://www.337skymaster.com/messages...1&d=1649733211
When you fill the system, you have to back pressurize it to get all the air out. I used a garden sprayer with purpose built connector that connects to the bleeder from aircraft spruce. I also got their high end bleeder screws to put in the calipers all to reduce the chance of getting air in the system. When you rebuild the master cylinders, the gap on the piston/lock-o-seal is important.