As a Registered Piano Technician I have had the misfortune of working
on a number of these newer mechanisms over the years.  At this point I
flatly refuse to look at them.  Every once in a while the local dealer
might ask me to glance at one that came in on trade, if I happen to be
in the store.  But, unless the problem is something really obvious, I
won't touch it.  The pianos themselves in these 70's vintage machines
were horrible.  My typical recommendation to the dealer is to gut them
and sell it for whatever they can.  That may sound sacrilegious to
some; but from an economical standpoint, they simply aren't feasible to
work on. 

In my experience the biggest problem is the plastic valve blocks.  By
design they were built broken.  The neoprene valves never seat well and
sometimes they even develop micro-fractures and leak through the
joints.  I don't recall who, but I think there was someone out there
selling replacements.  The quality of the pneumatic cloth was typically
not that good, and the cloth is often leaky even if it looks good.  The
suction boxes were barely adequate to play the pianos well when new; so 
at this point they are most certainly under-powered for the task.

For all practical purposes these systems need complete rebuilding,
which would have to include higher-quality retrofit valve blocks.
Again, the economics simply don't justify this.  If someone wants to
put all the effort and expense into a rebuilding job, it might as well
be done on a good vintage piano, not one of these things from the 70's.

I have found that, even if I can get one to play a little, I end up
getting married to it.  The owner will call you again and again hoping
you can patch it together.  Eventually they start turning their
frustrations onto you. After all you were supposed to have fixed it,
and if it keeps failing, that must be YOUR fault, right?  If you are
fortunate enough to have gotten it playing at all, consider yourself
lucky and walk away quickly.

Rob Goodale
Las Vegas, NV