"years, out standing in a field" is somewhat different than "years of being outstanding in your field".
;-)
gotta love commas.

Very curious indeed.
It is beginning to appear that the person checking the truck is at the very least unfamiliar with master cylinders that are kept full from a remote reservoir.
I wonder if the brake fluid level indicator lit up after this person removed the master cylinder hose adapter at an earlier time believing that was how to check the brake fluid level.
We had a guy in our shop years ago who was under the impression that the brake fluid hose from the dash mounted reservoir was nothing but a "breather hose" until I called his bluff on whether or not he had checked the brake fluid level when he allegedly did a minor PM inspection.
The ensuing "confrontation" with him revealed his lack of experience despite his boasting of "years in the field". Maybe a corn field.

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.

Thanks guys for the info. I went out myself with my mechanic to check the suggestions you all gave. Turns out he did not check the reservoir he took the cap off of the master cylinder behind the booster. The reservoir under the dash was actually empty. Thanks again.

Bad parking brake switch,probably the little metal arm broke off the micro switch.........l

You might have a faulty brake fluid reservoir sensor. If you unplug it does it it goes out?