>I'm replacing the rear heater core and heater valve in my vanagon,
>can anyone tell me can I do this without draining my entire cooling
>system?

Yes.

>If so I plan to clamp off in & out hoses and replace core.

That's the plan.  Use plenty of old rags to catch any spillage.  And get
a new gasket to go between the core and valve assembly.

>I
>assume the white plastic screw is a bleeder valve, how long do you think
>it takes to bleed system, and should I also bleed from the radiator
>bleeder valve?

The white plastic screw is a bleeder screw, that is actually what broke
in my Westy.  I had a complete new core on hand so I swapped it in using
the method above.  I have since extricated the remains of the broken
bleed screw in the original core, and replaced it with a new bleed screw,
o ring, and gasket, and the rehabbed core is giving me fine service in my
87 Syncro.

I did not open the bleed valve in the heater when doing the after
replacement bleed ritual.  I bled the system using the radiator bleed
screw and a steep driveway.  Worked well for me.

Good luck with your core/valve replacement.

Best wishes,

Tony

Anthony L. Mourkas
Hampden, Maine, USA
1984 Vanagon Westfalia
1987 Vanagon GL Syncro
1991 Jetta Eco Diesel