I did a similar thing by doing a bepec domestic gas foundation course over a 12 months period, during which time I gained on the job experiance and built up a portfolio with a service engineer that works for CHN in the west midlands. I then took my acs after passing the bepec course and then...
Might seam like a stupid question but did you close the isolating valves on the flow or return pipe under the boiler and if you did did you open them up fully.
easy to test get a pint glass of water and put the end of the vent pipe into it if water gets sucked in then thats your problem have you changed the pump recently.