The advertised life for the dessicant bag inside most automotive dryers
is about twenty minutes if the manufacturing assembly area was extremely
dry. The dryer media is a hydroscopic silica gel that adsorbs water. In
order to restore the anhydrous state of the dessicant you have to apply
vacuum to the container and raise the temperature to above 150*F to get
the water to release from the media. Some old time shops would drill a
hole in the side of an autoclave so they could pull a vacuum on the
dryer bottle while the oven ran a sterilization cycle. You can pull a
vacuum strong enough to collapse the hoses in the system and still not
get a saturated dryer to release the water it holds.

Now to second guess your mechanic. Although you may have holed the
condensor or blown a hose or suffered some other leak in the system, the
component that is by far the most common source of a slow leak in an
850, ?60, ?70, ?80, ?90, is the evaporator.

Bob

You *MUST* replace the reciever/dryer when the system has been opened
for more than a few minutes, if you don't then you may well be replacing
the compressor and expansion valve as well as having the dessicant
pellets flushed out of the entire system.

The part is like $30, any mechanic who doesn't automatically replace it
shouldn't be working on A/C systems.