What is the Lower Section? Where is it located? What does it do?
Before we can begin, we have to get a few terminologies straight. The bellows that are connected to the foot treadles (or pedals) are called the Exhausters. That's because they 'exhaust' (or pull out) the air from the player mechanism. Connected directly to the Exhausters (via a trunk or channel) are the Reservoirs. Some units have a single reservoir and I have seen units with two reservoirs AND an Equalizer bellow which is attached to one or both of the Reservoirs. Check out the rough drawings by Clicking Here
Testing the Lower Section is actually pretty easy by comparison to some of the other components in a player piano. In a nut shell, remove all the hoses connected to the lower section, tape all the exposed nipples (or flanges) closed with a good duct tape and start pumping. In an excellent lower section, the vacuum will build up so rapidly that after two or three pumps, the reservoirs will be completely collapsed and further pumping will be extremely difficult. Forcing the unit further could cause damage.
On the other end of the spectrum, if you pump and pump and the unit never seems to 'lock-up', then you've got some serious leakage. Most leakage can be traced to the bellows cloth or the flap valves on the back of the exhausters. If the Reservoirs collapse most of the way but take more than just a few seconds to reopen, then count how long it takes. If it takes more than 12 seconds to go from fully collapsed to fully open, then the unit is in fair condition. Rebuilt units will hold vacuum for over a minute in some cases. Most manufactures required a minimum of 30 seconds before the lower section was installed.
Sometimes, cracks develope in the wooden parts and the wood (plywood) literally 'comes apart'. This is not always obvious to the naked eye unless you hit the wood with the fleshy side of your fist. Then you will hear a 'slapping' type sound. Cracks also develope at the seams where various pieces are glued together. This actually happens quite a bit in 1920's units. It's due to their age. All wood continues to dry and shrink for many, many years. At some point, if not well sealed, the seams split open and let the air 'in'. Since the unit stores vacuum, sealing these types of cracks with Phenoseal or other such sealers is a reasonable 'quick fix'. However, it is better to remove, clean, reglue and clamp this type of damage.
