Very good question! This is a bypass. No anti-freeze, should come out.
By unloosening this a turn or two, you are making  a connection
between 2 areas of the system, in order to purge out the air. At no time
should this screw release fluid. It simply is a temperary by pass.

            Eric                   86-VW4x4
            vw4x4@fyi.net          72-240z
            Pittsburgh, PA USA     1936-Chrysler

On Wed, 8 Jul 1998, Brian Doss wrote:

> my '88 has a "bleeder screw" which is a knob on top of the thermo housing. In
> the Bentley book, it says to open this knob by turning counter clockwise
> before and during filling the cooling system. Well, at no time during the
> process did I see any coolant escape from this bleeder, so what is it's
> function, and should coolant escape from here in a properly air-purged system?
>