I noticed that besides the coolant bleeder valve on top of the radiator
and the one in the engine compartment, there is also one on the
secondary heat exchanger under the rear bench.

There is no reference to this valve in the Bentley manual. Picture
B25-270 page 40 of the Volkswagen owner's manual does show it but no
reference is made to it.

Are there any special instructions for this valve?

Hans Brouwer
Cumberland, Ontario
86 Westfalia