That valve never seemed to close properly on mine either. I always found the heater matrix still getting hot in summer.
It was mainly, due to cable clamp problems, but even if I went unter the bonnet at moved it all the way to the cold end manually,
there was still some heat getting through.
Sid
----- Original Message -----
From: "droopsnoot" <mechanicalhelp@vauxhallviva.com>
To: <mechanicalhelp@vauxhallviva.com>
Sent: Monday, October 09, 2006 9:49 AM
Subject: Re: Heater radiator - '74 Magnum
Well in theory as soon as the heater valve is shut off, the water should start cooling down, especially if you use the fan.
There's bound to be some seepage unless you've got a 100% seal, but it should cool quite well.
I don't have the cable connected on mine, so it's always blowing warm air, but that's mainly because I can't find a suitable clip
to hold the outer.