boiler lockout error after refilling open vented central heating

[tellme_why]

Hi, I have drained the CH HW system (traditional open vented system) and refilled.
Bled air from ground and first floor.
It seems there is some air in the boiler -> cylinder pipes (boiler on ground floor, cylinder on first floor) - I can hear it when pump running.
Boiler locking out; how can I clear that air?
There is no bleeding valve in the pump, but there seems to be a tap at the bottom of the cylinder - but I don't want to touch it as it's old and it may break (just happen to a radiator drainage valve)
Not sure what to do..

 

[SithLordHood]

Share some photos of the tap at the bottom off the cylinder.

 

[tellme_why]

Hi Sith, thank you for your reply; it seems that the problem is now solved.
Air in the big horizontal pipes between cylinder and boiler cleared after some pump running and 1h rest.
Boiler not longer locking out.. finger crossed...