asterisk users - Nov 2009 - channel destruction after a transfer call

Hi,
we're using Asterisk 1.6.1.1.

We've got a problem during a transfer call concerning the destroying of
the unused channel.

At the beginning we have a call A -> B (A and B being two user).
With the AMI interface we ask the redirection of:
- case 1: the caller (user A)
- case 2: the called (user B)
to the extension C (not a user but a simple "Park()" dialplan
application).

At the end of the redirection, we noticed a different behaviour in the
two cases:
case 1: A -> C and A' -> B (when C answer, the unused channel
A'->B is
immediately destroyed)
case 2: B -> C and A -> B' (when C answer, the unused channel
B'->A is
destroyed after a variable time by some kind of garbage collector)

Is this the normal behaviour, or it depends on our specific redirection?
If C were a user instead, we noticed the same behaviour in the two cases
(in both cases, the channel is immediately destroyed).

We need the immediately destruction of the unused channel, to
immediately reuse it to place another call.

How can we make asterisk close immediately the unused channel after the
redirection?

Thanks a lot!