Originally Posted by
DBMandrake
If your bypass radiator is AFTER one of the zone valves, it's not a bypass radiator as that zone valve would isolate it.
I think given what you describe disconnecting the zone valves and fitting fixed heads (you can usually get a head with a manual knob to lock them fully open) is your best bet. At a pinch you could set them in the latched filling position but as this only opens the valve about half way it can cause some flow restriction that is probably best avoided.
I would probably manually close and re-open the valves once a year just to make sure they don't seize up.