Bolier losing pressure

[voodoocars]

Hi, a while ago i posted on here about my Vaillant turbomax combi having trouble with the DHW heat exchanger. Had it removed and de-scaled as suggested and the pressure relief valve replaced as it was knackered - everything was going ok until now.
I now have a pressure drop from 1.3 bar to zero over a week period. The DHW is not as hot as it should be again (prob due to scale again because of re filling) so I know about that.
I have checked everything to find the leak to no avail - relief valve-dry, rad valves-dry, joints-dry. Everything I can think of in fact. Am I missing something?
When I top it up and run it I can hear air rushing round the boiler.Th heating works ok.
Any ideas...

 

[marleyco]

not the best at boiler diagnosis, 2 thoughts are expansion vessel ruptured inside or simply a pipe leak most likely under floor

 

[Billy Bob Bob]

when you say prv dry have you checked the external discharge pipework?anything can cause this in a sealed system if you arenot gsr you need to get one of my colleges to look in side the boiler and check out things like aav [automatic air vent]and genral water connections