I would suggest pulling all the wheels and taking a good close look at the braking system. Peel back the dust cover rubbers on the wheel cylinders. If you see any fluid there, it is time to change them. Check the condition of the brake hoses as well, they should be flexible and free from cracks.. Regular brake fluid attracts moisture, the steel lines could be due to be changed if they have been in use for a long time..
I changed my truck to a dual master cylinder also. I like the appearance of a stock truck, but the master cylinder is under the floor so the conversion is out of sight..
Your truck most likely has DOT 3 in it, you should be safe in using it to refill the system. As mentioned earlier, if the master cylinder ran so low as to let air into the system, they will have to be bled.
If this is the same truck that you plan on putting the Patrick's 3:55 gears in, you had better switch priorities. Traveling in the left hand lane with an empty master cylinder is not what I call Happy Motoring.
