Checks cause hammer bounce ?

[RicB]

Hi folks

I ran into a symptom I haven't identified before.  I was bedding an 
older Steinway D today, and made sure all three rails were good and done 
in the right order... yet I was still getting too much hammer bounce in 
the bass section on a hard blow.  The rest of the piano was just fine... 
just the bass section had this problem.  Started troubleshooting by 
first lifting the whippen up with my finger and hitting the key hard to 
see if there was any bounce... nope..  Then I lifted the hammer with a 
shank hook and hit the key hard... still no bounce.... Then I noticed 
that when I lowered the shank enough for the back check to make contact 
with the hammer tail... the neighboring hammers started to bounce.  When 
I let the hammer go into full check the full effect of neighboring 
bounce kicked in.

I haven't managed to pounce out what the cure for this is.  Checking 
distance is 15 mm... action is reasonably well regulated... drop is a 
bit deep but letoff, dip and aftertouch amounts are all well within good 
tolerances.

Any advice on the matter is of course very welcome.

Cheers
RicB