The more closely I’m looking at this in trying to address its leakiness, the more similarity I see with the 34 key Clavietta. It has to be Yamaha’s answer to the Clavietta. Too much air is leaking to make meaningful sound comparisons, yet this Pianica is to all appearances more solidly constructed than a Clavietta and in much better condition that the three of those I have examined.
If you look at the last photo above, you see the chambers for each individual key. When the key is depressed, the plunger is raised (from this bottom side view), and air escapes and hits the reed side of the reed plate. By placing my lips over the opening, I can tell by blowing and sucking that the seals on all these plungers are fine — no air is escaping in either direction. So, unlike the situation with an old Clavietta, there’s no need for replacement of decomposed pads or washers, which is very good news. The plunger rod in each case runs from the key through a metal plate which is less than 1/4 inch (I’m estimating) from the bottom (from above in the picture) of the individual reed chamber. This means that the main air chamber that runs the length of the Pianica is very thin. Air must be escaping either from this chamber or from the vent end of the spit valve mechanism or thereabouts.
Opening the main air chamber will require removal of all the keys. I think some air is escaping from the vent end of the spit valve mechanism. I’m just thinking out loud here, but someone may have insight to offer. I’m still experimenting and don’t want to be hasty about removing all the keys, etc.