Sostenuto Puzzler

[Kent Swafford]

I'm not usually one for puzzlers, but I think this one is interesting; 
I'll post later why I think so.

This week I was getting a Steinway D ready for a concert that was 
tonight. I put a fairly complete regulation on the action. I found that 
the sostenuto was working poorly and regulated it, but the pianist 
complained of a cracking sound upon release. This is a first-rate pianist 
and I knew that the complaint was likely to be valid. Here is what I 
found:

Hold some notes down, engage the sostenuto, play other notes and engage 
the damper pedal, release the sostenuto pedal. At this point the monkey 
does not descend, but stays up. When the damper pedal is released, the 
monkey comes down with the "crack" that the pianist complained about. (I 
noticed that when the damper pedal is keeping the sostenuto engaged, one 
can engage the una corde with no effect on the sostenuto; the monkey 
stays up.)

How can this be? How can the damper pedal keep the sostenuto monkey and 
blade in the engaged position?

Kent Swafford