Added Radiator - now weird air gaps?

I added a new raditaor to a garage conversion today, probably not the textbook way of doing it, bit I ran 10mm hep pipes to the next nearest rad in the house and used a T to split the feed and return.
Anyway, rather than drain the system to fit it, I pulled apart the existing joints , (having turned off the heating first) to quickly plug it without losing too much water. It worked without loosing much water. But when it was installed nothing would bleed from the rad, until, I found, I went and opened the bleed valve on an upstairs rad (2nd floor). Also there was a racket coming from the burner, air gaps causing hammering etc.
this was all fixed by opening an upstairs bleed valve, but now, although the ground floor radiators are all bleeding fine, it seems nothing at all comes out of the 1st or second floor radiators. Even though they're hot. I can't work that out!

Why won't a hot radiator bleed any air or water at all! No movement at all!

I guess I've got some weird air gaps, but how to I release them?

Thanks!

Do you have a sealed or open vented heating system

It's a weird system - a Gledhill boilermate. There is a little cistern in a storage cupboard on the floor above, I guess to keep the system topped up. I checked, and it's still full, I can't see any valves that are off or anything

Did you connect the 2 pipes to your new radiator to 2 seperate pipes from existing radiator, you didn`t just tee your new rad into one pipe?

Yes, both the feed and return were tee'd to run to both radiators. The new ones work fine, this morning they're both hot. But air gaps mean the 2nd floor rads need bleeding but won't let anything through.

Aha! I found the little dial on the small red tank above the water tank was reading zero - and found the little isolator that allows it to refil - I guess this is how I refil the heating's water??

Do I leave that valve in the open position while i go round and bleed the system, or let it in bit by bit as I bleed them?

bit x bit