Fitting towel rail in place of radiator

[kennymcd]

I want to fit a new towel rail in place of radiator in bathroom. I have a Combi Boiler system.
I have closed off both valves on radiator, drained the radiator and removed radiator from wall. With the radiator off I opened both valves to check that the water supply is present but no water came out of either. Is this normal with a Combi boiler and how do I get the water to come back when I have fitted the towel rail. Thanks

 

[frodo]

you are air bound, bled the system , from the return side