Heat on to 1 zone with no thermostat installed???

Last night the heat in my bedrooms came on. I removed the thermostat, no change. Went down to the basement and changed my zone valve head (TACO) and still have the same issue? What else should I look for?

The boiler is an old weil Mclean the two zone valves are TACO 571-2. Heat has been off for several weeks if that matters?

The zone valve is stuck in the open position- whether that's field-fixable without replacement remains to be seen, but the theory of how it operates is well described in the manual, and may help figure out how it failed, and whether it's reversible/repairable.

I was hoping not to have to pull that valve out and drain the boiler... Oh well, I'm off work tomorrow and I'll pull it out.

Thanks!