That is an apocryphal myth that has circulated for some time, helped
no doubt by the manufacturers of auto parts.

Yes, the dryer can become saturated by exposure to humid air.  But the
process would take many hours to days if you just uncapped it and let it
sit.
Remember the construction of the dryer (or accumulator if you have that kind
of system) is such that the coolant must be circulated inside of it, through
the
dessicant.  And the idea that a dryer that is buried behind a half meter of
AC line would absorb significant moisture is even more absurd.  Once the
A/C line reaches atmospheric pressure, the remaining refrigerant is not
going
to come out of the line if you have capped off the lines with tape.

The biggest problem here is that the average home mechanic does not have
the tools to do this kind of operation with any success.  Here's what needs
to be done:

1) Take the vehicle to an A/C shop and have them reclaim the refrigerant
and let the line pressure drop to atmospheric.

2) Disconnect the A/C lines.

3) R&R the radiator.

4) Replace the O-rings on the ends of the A/C lines disconnected.

5) Reconnect the A/C lines.

6) Evacuate the system with a vacuum.  Repressurize the system with
R134a.  Check for leaks.

Items 1 & 6 generally cannot be done at home.

A complete waste of money and it just gives 2 more areas that you
could possibly not get a good seal back on, and more leaks.  Also, if
your really convinced that your dryer is full of water, all you have to
do is bake it in an oven at about 225 degrees overnight (above the
boiling point of water, and below the flash/firing point of the refrigerant
oil) this will drive all the water out of it and it will be good as new.
The
oven might stink afterwards, though. ;-)

Ted