See pic below: This is the expansion vessel that loses charge. I pumped it up from the Schrader valve to solve the problem. This is a common fault on Vaillant Ecotec boilers and is caused by the expansion vessel losing its charge.

If you have to keep repressurising your Vaillant boiler you should get that problem found and fixed as soon as possible. A leak could be anywhere on the full system, boiler, radiators, or pipework under floors. Leaking radiator valve The pressure rises when the heating is on, so you will not get the proper reading until it has cooled down. This is usually below the boiler but there could be an external flexible filling loop anywhere on the system, sometimes hidden away in cupboards or behind boxing.

However if the system is under pressure then my understanding is that of course water would come out of the air valve and the person doing the job is trying to con me into getting unnecessary work?

This means the expansion vessel on the Vaillant boiler might not be big enough and can cause problems with the pressure. If the pressure drops, it means the water is getting out which should not be happening. Low pressure on a Vaillant boilerThis is most likely an expansion vessel problem, if so, you will need to recharge it or replace the expansion vessel if the diaphragm inside is split. I'm no expert at all but from reading lots it points to the expansion vessel either needing a recharge or is failing/failed. I see that you can test the Schrader value for water. I also read that vessels last around 10 years, which if true means we're fairly close to it. What I have learned and understand so far is that there should be a release of air from the valve when activated when the boiler is switched on or off.