Have you followed where the brake pedal goes to? Some of the larger rigs had a cylinder resivour up high on the fire wall and some had them on the upper portion of the main frame near front drivers wheel.
Im with the good Wolf man on this. The linkage which gets pushed when you step on the brake pedal eventually is what pushes the piston into the master brake cylinder. About always located on the upper firewall on the drivers side somewhere. NOTE in years of experience on old trucks, if the master cylinder is empty its OFTEN because one or more of the brakes wheel cylinders have leaked and need replaced. If you look at all the wheels on the inside and at the bottom there's nasty looking brake fluid the wheel cylinders are leaking. Brake repair is NOT for the timid or inexperienced. If theres air in the system it will need bleeding and leaking fluid gets the drums or brake shoes all messed up yukkkkkkk. If you find the master cylinder theres a top cap which you remove and fill with brake fluid and see if they will pump up and hold but the master or wheel cylinders may be leaking and it will get empty again........This is too complicated to describe here, so get help if needed.
Lisa,did you find your master cylinder?Bet it was where I said it would be in that message I sent to you.
You should see it on the inside of the frame on the drivers side.It will have one brake line and a cap on the top to fill it.I would just follow the brake pedal linkage to find it.I've seen M/C's mounted under dash as well on these models.Good luck.