Vehicle: 2007 Toyota Tacoma 4WD v6 6 speed
I found this tutorial on pre-bleeding but it is generic (not vehicle specific) and I think there are differences between the cylinder it uses and Tacoma's, which makes it hard to translate the instructions practically. The main difference is, I think, that his cylinder has the tank mounted on it directly whereas Tacoma's has a hose that feeds the fluid to it from the brake fluid tank, making it difficult to keep the tank on and topped off like it is suggesting.
As can be seen in the picture below, my master cylinder has an intake nozzle that the rubber hose gets clamped onto. I experimented trying to pour the fluid directly inside (by taking the the white nozzle cap off) and the fluid was coming inside very, very slowly, as though the nozzle was clogged, with tiny bubbles coming out. I also wanted to see whether there was cloggage or something (it's a brand new cylinder) cause I was trying to bleed it while it was installed (I took it out since) but the fluid was just not coming in. I also tried venting the slave cylinder but, as I was pumping the clutch pedal, nothing was coming out of there after some initial emulsified spray.
I also tried to see what happens if I take out the clutch line out of the cylinder while it was installed, while leaving the intake hose in. I thought I was going to get a sh*tstorm of fluid coming out of the line valve as I pumped the pedal but there was almost nothing coming out. So I started questioning whether there was actually fluid coming through the intake hose from the reservoir but there was (there was no cloggage in the intake hose coming out of the firewall, i ran a smaller hose through it and it went through no problem).
No mater how much I would pump the pedal (and the cylinder plunger was properly mounted on it), the cylinder seemed not to be sucking the fluid in from the hose. Which is perhaps explained by the bolded statement above about the experiment with pouring the fluid in directly. However, this is just a speculation.
BONUS QUESTION: How is the master cylinder nozzle designed to allow the fluid to only come inside without pumping it out in reverse upon the pumping action? Does that design explain why the fluid is so slow to come in (as described in the experiment).