Airlock in twin radiator.

[Bogart]

Twin rad with only waterways at bottom. Each half has its own bleed valve. Back heats up front does not surely must be a blocked bleed valve?

 

[king of pipes]

Possibly ? or a blocked waterway in the rad, try turning all your radiators off except that one and let the system run it may clear it . Then reintroduce the other rads till all back online . Kop

 

[townfanjon]

Find a thick needle or similar , and give it a gentle poke in the hole lol

 

[Bogart]

My sentiments exactly.

 

[moonlight]

Find a thick needle or similar , and give it a gentle poke in the hole lol

Turn both ends of the radiator off before you poke the needle in the hole incase you drop the vent piece. Then as king of pipes suggests.