can't remove air from system

[bosh34]

Hi,
Just wondering if anyone can help. There is very little air in our upstairs radiators.

When I try and bleed them a little air comes out but then the hissing stops. I've tried with the filling tap open and closed but even with the tap open air stops coming out quite quickly. If I turn the system on with the valves open some more air is pushed out (presumably by the pump) but then this stops very soon as well.
If the system is then switched off a little more air escapes but not much. I've tried opening all the valves on the upstiars rads and repeating this a process (switching the system on and off a few times) but it still doesn't really seem to have any effect. Does anyone have any idea what could be wrong?

 

[stiler]

You could try fitting some auto-bleed valves to your radiators
http://www.nlbengineering.co.uk/
£5 from PTS.

I would also fit a Spirotech Spirovent domestic Deaerator
http://www.spirotech.co.uk/en/products/

It's up to you if you want to fit a a Spirotrap Dirt separator at the same time or even the combined Deaerator & Dirt separator but it would help maintain your system at it's best.