britony chaffoteaux 100 combi?

ive had some problems with a boiler ive been working on. the pressure kept dropping, i found that water was coming out of the pressure relief valve,there was no pressure in the expansion vessel, i changed the expansion vessel and pressure relief valve and the boiler seemed fine,after a week or so the customer called and said the pressure is up and down and shes had to refill the system again. there are no leaks on the system. i noticed when i changed vessel there was sludge in the hose, blocked basically, could this be the prob. any advice would be helpfull, thanks