My 93' 2WD pickup's clutch has decided to stop disengaging over the past few days when I push it in all the way. Also when I'm idling, I'll put it in first (with more resistance than usual) and with the clutch still pushed in there's a rumbling, as if the clutch is still partially engaged. Clutch in in neutral is fine however.
It has a fairly new clutch that's about a year old, and even then the truck sat for 7 months as the motor was dead until I could afford to put a new one in. Plus when it is in gear it grabs really nicely. So I'm pretty sure it's not the clutch.

I thought it was a master cylinder or slave cylinder, but those were changed when the clutch was done. I did the master first but then a shop did the clutch and I'm 90% sure they also did the slave.
Either way, I looked at both and I can't find a leak, though the fluid reservoir is about 1cm lower than the max line. Is that enough to cause issue or be evidence of a leak somewhere?

Yes, I checked the pedal bracket that notoriously breaks and it's 100% intact. No movement at all when the clutch is in action and I scoured it to make sure it's not fractured.
What do you guys think? I just got it back from the shop after a valve adjustment and I really don't want it to go back

Edit: I had a friend push the clutch in while I watched the slave cylinder and it was going back and forth as it should. It looked to be the distance it needs to go but I don't know much about that. Also I should mention that there's resistance somewhat intermittently and of varying degrees. Last night I fired it up to go somewhere and it took two attempts to get it into first. Lastly, it shifts totally fine when the truck is off.

 

Came in here thinking it was another linkage from the pedal to the master issue, mine broke into pieces and had the exact behavior you described but you checked that already. As long as there is enough fluid in the res for you not to pull air it should work just fine, you can try topping it off but I doubt it makes a difference.

Have you tried a simple bleed to determine if it changes anything? Was the throwout bearing replaced with the clutch? Any chance moisture/water got somewhere it shouldn't while it was sitting waiting for a motor?

 

So not long after making this post I went out to look at it again just to definitely make sure I'm not missing something, and turns out that the sun was in a different position than from when I checked that morning, and thanks to that I could see that the fluid level was actually below the minimum line (it was higher up on the res than I thought it would be) and that there was the shine of a little bit of fluid around the res of the master cylinder. I had my friend pump the clutch while I looked at it closely but couldn't see any new fluid coming out. I think it's a leak, just a really small one. I did top off the fluid and that seemed to help a bit. Shifting was still odd but wasn't hard to shift as often as before.

I picked up the last master cylinder at Auto Zone today and will install soon. Will update when I do.

As for your questions COMTB, I'll definitely bleed it when I swap the master. Honestly I have no idea if the throwout bearing was changed or not when I had my clutch done. As for moisture/water, I doubt it. It just sat at my mom's place and no one ever messed with it the whole time. It through over a Georgia Summer though, which are notoriously humid, I guess there's always that chance.

 

Sounds to me like air trapped in the clutch hydraulic system. The previous day's clutch pedal pumping could have pushed some air out of the system, but if you have even a small leak between the res & master cylinder body, it can & will draw some air in thru that leak point.
Unless you've got a vacuum/power bleeder & a friend up top to keep the clutch master cylinder always filled with fluid on the fly, it can be difficult to get all the air out of this system from the bottom, i.e. slave cylinder bleed screw. Air likes to rise, not go down.
In the past I've gone thru two large bottles of fresh fluid trying to get all the air out thru manual bleeding, & still have a bad pedal & bad shifting. But I found a trick that works on these systems.

You just need to find a way to hold the clutch pedal down to the floor & let it stay that way overnight. This allows any air in the system to rise up & "burp" out in the res on it's own. I've seen a res go from completely full to half full in the morning, there was that much air trapped in there.
This has nearly always worked for me.

Ways to hold clutch pedal down? Broomstick or 2x4 cut to correct length to jam between floored pedal & seat; cinder block (concrete block to hold pedal down; telescoping car wash brush handle.
I use that last one because I happened to have one lying around, and it has a twist-lock device on it. Easy.