Since it has set for any period of time you will have to go thru the whole brake system rebuilding or replacing everything from the master cylinder to the wheel cylinders. brake fluid draws moisture and corrodes everything as well as the rubbers disintergrate over time. and the steel lines rust from the inside. The extra time and expense WILL SAVE YOUR LIFE!!! I got a rebuilt Hydro-vac for my Ford F-600 from BEPCO, Winston Salem NC 336-760-0740 or 800-735-1154 came FedEx in about 2 days.. Save the box and styrofoam for your core return.. I do NOT know if there is a way to add a dual master cylinder to this type of system. I have not done so on my F-600. The control on the steering column MAY(??) be a hydraulic brake lock,in addition to the parking brake. A lot of fire trucks had them..I doubt if it is a trailer application valve as vacuum braked trailers were long gone by this era.. Your hand parking brake applies to the drive shaft right behind the tranny, some are drum and some are disc. First soak the linkage good with panther pi$$ and the see if you can adjust it up stronger if there is any lining on it. If what you are describing as a "scissors lift" is one which lifts the whole body straight up then dumps it, SAVE IT as they are scarce.. They were used only by coal delivery trucks from the 20's to the 60's and a few roofing companys. You have a good looking truck.. Keep the pictures coming!! You will find plenty of help on this board. Brocky