changing radiator valve

[boy01zone]

I'm trying to change a rad valve without draining the system.
I've tried using a Drayton easy drain bung, sealing the header tank and back fill pipe, turning the opposite end rad valve off, and CH turned off, but when I open the valve in question water just flows fast.

I would appreciate any advise

 

[seco services]

how long did you let it flow for ? useally get about 1/4 pint before it stops.

 

[boy01zone]

the rate the water was flowing was too fast, it was like a river as if under pressure, yet CH was turned off.