As I couldn't find much help anywhere else and everything everyone suggested to try didn't help, I'm making this post as a reference for others who are having the same issue I did, not being able to build pressure while bleeding the clutch. Please feel free to add anything that helped you. This is on my 2004 G35 coupe.

TL;DR: I replaced my slave cylinder, couldn't build pressure while bleeding, slave wouldn't move clutch fork at all. Things to try that helped me:

1. Bleed the slave "by hand" before putting on the car; Put fluid in it and pump until all air is out, make sure plunger is fully extended before proceeding. Attach clutch line to slave before bolting slave to tranny.

2. "Bench bleed" on the car: Using this one-man bleed method: detach master cylinder rod from pedal, use screwdriver to push rod all the way in, repeating until all air is out of the system.

3. Close the bleeder, attach mc rod to pedal, ignore conventional bleeding, just pump the sh*t out the pedal.


If you want the details:
Long story short, I replaced my slave cylinder mostly out of "preventative maintenance". When I took the old one off, I left the reservoir cap off leaving all the old fluid to drain out. Put the new one on, and just tried filling and bleeding, following the book step by step. The problem was fluid would barely move through the system, and no pressure would build at all. I tried backing the mc rod out of the pedal like everyone suggested but it made no difference. I was told air is just still stuck in the line and to just keep bleeding, but I did conventional bleeding for about a half hour straight with no change so I decided to ignore that advice. I tried vacuum bleeding, I tried "pressure bleeding" with a fluid pump as I saw two different people say they tried that with success, but nothing worked.

I said f*ck it, put the old slave back on to see if it made a difference, same exact result. Put the new one back on but this time, left the reservoir cap on the keep the fluid from draining. Pumped the slave by hand, putting fluid in it until no air bubbles showed and it was full when the plunger was fully extended. Attached the clutch line to the slave BEFORE I bolted the slave to the tranny. You'll have to fight it a little to get the bolts lined up but it's still not too hard. This step probably isn't even necessary but it can't hurt.
After the slave was bolted up, I didn't even do any bleeding, I immediately had pressure in the pedal and could go into gear perfectly.

However, I went to bring the mc rod back in to where it was before I extended it out, and for some reason when I did that, the system lost all pressure again and slave wouldn't move again. My plan then was to take off the mc and bench bleed it, but as I have no flare nut wrenches, when I tried to remove the line from the mc, it was so tight I was afraid of stripping it, decided to try just doing it on the car. Using the above one-man bleed method, attaching a hose to the bleed valve, and having the other end submerged in liquid, leave the bleed valve open. Detach the mc rod from the pedal, and use a screwdriver to push it all the way in repeatedly, making sure you don't let the reservoir run dry. I tried doing this until there were no bubbles in the tube coming from the bleed valve, but after putting a large amount of fluid through the system there were still a lot of air bubbles coming out, so I assumed it was coming from the bleeder (maybe try putting some teflon tape on the threads to prevent this. I didn't because I was lazy, but still made out fine). I closed to bleeder, pumped with the screw driver a couple more times, pressure started building immediately to where I couldn't pump by hand anymore.

Reattached the mc rod to the pedal, and had just enough pressure to move the slave. The engagement point was right as I let the pedal off the floor, but it was still enough that it went into gear smoothly. At this point I could drive it and was going to take it to my mechanic who actually knows what hes doing to properly bleed it. But after driving it for about 40 minutes, right before I got home, the engagement point was much higher and the pedal overall felt much better, I guess air just needed to get out of the system. There was still a bit of mushiness at the top, but that too went away after another day or two of driving. Two weeks later I've had no problems and it's been the same if not better than before I got myself into this mess.

So hopefully this helps someone out and doesn't have to spend the 8 extra dismal hours that I had to go through for this one hour job.
And if anyone has any comments on my terrible mechanic practices or anything to add that helped them, please do so.

 

First, I strongly recommend you get the vehicle up in the air on a lift or stands, start the engine, shift to first gear and hold the clutch depressed while you watch to see if the rear tires still spin a little. From what you described it sounds like you probably still have air in the system and you might not be FULLY disengaging the clutch, you're going to cook the clutch prematurely if you don't have 100% disengagement. It might not be enough to move the 3500 lb car forward but might still be slightly engaged. Testing on a lift will guarantee it's actually 100% disengaged.

ALWAYS bench bleed a master cylinder before installation. They make plastic fittings you can screw into where the brake lines / clutch line attach so you can adapt directly to clear plastic hose, use about 8-10" of hose and have them drain directly back into the reservoir (google search brake master cylinder bench bleed kit). You definitely want to keep pushing fluid through it until you stop getting bubbles.

Don't actuate that piston quickly, smooth full strokes is the trick, watch the clear hose, once you stop getting bubbles continue to pump fluid like 5-10 more times. Usually it takes 30 strokes or so to completely get the air out.

Adjust the pedal according to the FSM, the clevis pin should float in the hole on the pedal and not be adjusted too tight/loose. I know you can get better pedal feel by adjusting it for quicker engagement but it works the master cylinder piston in a range it's not really designed to.

Don't use a power bleeder like the Motiv kind that pressurizes the reservoir, you can dislodge the o-ring on the piston then you're stuck disassembling the master cylinder and replacing the front o-ring.

The OEM pedal feels really bad, I know it's fairly expensive buying the RJM pedal but honestly the two best mods on my car are the RJM pedal and the B&M shifter. They just make the whole driving experience FEEL so much better.

 

Also, if you adjusted the clutch pedal throw you will also need to adjust the SWITCH on the pedal.

 

Agreed on getting the rear in the air. At this point I'm confident all the air is out but it's still good practice.

I did the clutch pedal spring removal "mod" awhile back and it's a much better pedal feel, I have no complaints, especially being free.

Also why would you need to adjust the switch? The clutch bottoming out is from the pedal hitting a stop, not from the throw rod bottoming out, at least as far as I can tell.

 

Maybe you can back bleed from the slave up to the master with a syringe?
I've had to do that on a few motorcyle clutches and front brakes.