ok i'm a bit confused....

there is no cap on the master cylinder on this truck
it is a plug with a hose nozzle that hooks to the hose that runs to the reservoir... that's it.

so basically whatever he told you at first was well.... you get the picture...
he just didn't check the reservoir which is generally plainly visible underneath the left hand side of the dash. In some cases i have seen it mounted behind a cover there but usually it is easily accessible for ease of service.

Hope he did not charge you for the return visit as it was his oversight the first time.
And since the reservoir was empty did he check the rest of the system for leaks? The fluid went somewhere. Just refilling it is not going to repair the problem causing the low fluid ;o)

pull the floor plate and recheck the master cylinder and see if it is wet where the rod enters the piston area(the boot will have to be pulled back to see inside it)... if it is then the master cylinder is leaking and should be repaired/replaced.

also check the wheels on the inside at the backing plates, if you see wet fluid around the wheel or backing plates on either side then you have a bad/leaking wheel cylinder.