You HAVE determined that the pump is not pulling vacuum?
How is pedal travel? If there is a lot if travel, check fluid level.
The check valve is very inexpensive, if you have any questions about it, just replace. To check the check valve, take it off the truck altogether, suck one end then the other, If you can suck one direction but not the other, it is good. If you can suck in both directions, it is not good. Also check the grommet for the check valve on the booster, they sometimes crack.