Q about bleeding radiators - help pls!?

You should just let the air out and stop when the water starts to come out.

Letting too much water out will be a problem if your system is sealed.

Check the pressure gauge on the boiler, it should; be 1-1.5 bar when cold. This will go up to 2 bar when it heats up.

If its less than this you will need to top up the boiler with water.

There will be a filling loop on or near the boiler with a valve that you can turn either with a small handle or sometimes a coin.

Here is a link to a picture of what a filling loop looks like and a video showing how to use it to repressurise your boiler.

http://www.worcester-bosch.co.uk/homeowner/product...

Simon H is right, but your system is not necessarily a sealed system. If it isn't, you can't have done any harm, unless the cylinder fails to heat up, so no hot water. ~In this case, the float valve in the header tank would have stuck. this is a fairly common problem.