To check the brakes, push the pedal down and hold it. If the pedal travels slowly to the floor, there is a leak in the system. If it is apparent the system is leaking, start off by checking at each wheel. Remove each wheel in turn and check for leaks. Look for leaks from the callipers on disc brake equipped vehicles, and wheel cylinders on drum brake equipped vehicles.
On drum brake vehicles, put each drum back on and feel how easily it pulls off. It is usually very difficult to coerce a drum to come off, but once you've broken the "seal of rust," they should only be slightly snug--if they come off too easily the wheel cylinders need adjustment. Out-of-adjustment wheel cylinders will also show up when you depress the brake pedal in the form of excessive pedal travel. Check the shoes for reasonably even wear and thickness and look for any signs of scoring in the drums. Try to turn the adjusters on the wheel cylinders. If they're seized, they will need replacement. You can get a rough idea if the drums are warped or not by putting them back on the truck and spinning them. There should be even resistance though a complete rotation.
Check all the steel lines for excessive corrosion or kinks. Flexible lines can be checked by "kinking" them back on themselves. If the edge that is in tension shows signs of cracks, it needs replacement.