Clutch problems?

I ran this truck for 16 years with no problems with the clutch. Slave cylinder had a bad leak, so I figured it was time to get the clutch redone. The mechanic installed a new slave cylinder and all the new components throughout. Clutch didn't act right after I got it back, seemed to release closer to the floor than I remembered. Anyway, about a year goes by and the new slave starts leaking again. The mechanic installed an entire new clutch again, because it was still under warranty. This time the clutch released really close to the floor. But I figured it was an old truck and I'd just have to live with it like that. It's been about a year and half now, and it started getting really hard to get into first gear and reverse. I checked master cylinder and hadn't lost any fluid. I pumped the clutch up at the pedal a bunch and it was easier to get into gear. I thought okay fixed. Wrong. It bled down overnight and lost no fluid but lost pressure if that makes sense. I thought maybe there was air in the line, so I bled the line repeatedly. Clutch worked well, next day went to drive to work and couldn't get into gear, bled pressure down again. Pumped it repeatedly, got it built up and went to work. The same thing happened at lunch and after work. I really don't want to have to pay to have another clutch installed, considering the clutch, pressure plate and everything else besides slave cylinder were still in pristine condition the first time after almost 145,000 miles. So, I'm sure the internal parts of the clutch are good. I took a picture of the slave cylinder through the hole in the side, and it looked dry and clean. After trying to suck out air from the line, the pedal is just flopping at this point. Would I be wrong to assume this is a master cylinder problem? Can the master cylinder go bad, let air in the system and not lose fluid in the process? I am hoping so, I ordered a master Tuesday and am waiting for it to arrive. Also, I've read there is possibly a bleeder valve in the hole above the one I took this picture from. I can't get my hand to the hole without a lot of extra effort. Is there a bleeder valve inside that hole? How do you access it? Thanks again for any help! Thanks for any help or guidance on this.