No brake fluid coming from back reservoir on new master cylinder

[gxc]

We ended up having the original master cylinder on the truck and we bled the breaks and everything worked fine except the master cylinder was leaking so we ended up replacing it. Now we've got three brand new master cylinders and I bench bled the third one we got and I still can't get any beak fluid to even come out of the back reservoir. It's on a 1972 chevy c10 by the way.

Can anybody help me out with this? Because we can't figure out why in the hell we can't get any fluid to come out.