What I was talking aobut....
The non-centrifugal unloaders use a check valve going into the tank,
and a dump valve connected to the head, controlled by the pressure
switch. As soon as the pressure is high enough, the pressure switch
opens to stop the motor, and opens the dump valve connected to the
head. So while there is enough air (idle) there is no pressure on the
head. Once the pressure is low enough the pressure switch closes to
start the motor, and closes the dump valve. The back pressure at the
head builds as the motor gets up to speed. Once the head pressure is
enough it pushes open the check valve on the way to the tank. The
amount of volume connected to the head before the check valve, will
determine how long it takes to build pressure and load the
motor/compressor. If the motor starts fast enough, it doesn't take much
volume. If the motor is starting up slower, then increasing the volume
will take more time before the compressor sees any back pressure, and
thus easier, longer startup time for the motor.
Not sure how Your centrifugal unloader is working....
If there is some volume between the compressor head and the check
valve, then increasing that amount of volume will allow more time for
the pressure to build up.

Now as long as you are adding volume between the compressor and the
check valve, you might as well make it usefull volume... Maybe a coil
would be helpfull, since that would provide additional volume, and also
help to cool the air, and thus reduce it's ability to hold moisture.
The amount of drying action for the air depeneds on how cool it gets,
so that the moisture can condense out.

As I mentioned elsewhere, check to see that in fact the comperssor head
is really unloaded before the motor tries to start up the compressor.
Your description of the problem, where the motor cannot start up the
compressor after the tank has been pumped up, points squarely at a
deffective unloader mechanism. That is exaclty what the unloader is
designed to fix.

HTH
Pete