THHBBPT!  ACK!

a new radiator cap, fresh off the dealer's shelf and not wet with
anti-freeze will make a disruptive farting sound as one blows strongly into
the hose barb.  do this at the dealer's service counter before you leave and
the laughs generated may get you an extra 10% off.

additionally, you will not be able to draw any air by sucking on the hose
barb. only until the design pressure has been reached should anything flow
out the hose barb.

i've had a bad cap on both my '85 and the '87 that allowed flowage both
ways.  me thinks various concoctions of "Radiator Powders" gum up the
workings of the cap :(

lastly, you all know that ethylene glycol is extremely toxic so this is a
BAD way to test a cap with anti-freeze in it.

-dan