My combi boiler keeps losing pressure. It's still giving hot water but I noticed the gauge on zero so called out a Hometree. They send an engineer initially just topped up the water. I explained that I knew how to do that but wanted to know why the pressure had dropped as surely something must be leaking. We went round the house checking the pipes etc but nothing is showing. He said to call him out again if the pressure dropped. 2 days later sure enough the pressure was gone so I called him back. We checked everything again but there's no sign of water anywhere. He says that it may be an internal leak and if so we need a new boiler. He's left me to keep topping it up and checking for leaks over the next week. I'm wondering if I need a second opinion? Not sure if hometree would send anyone else but I'm concerned that he can't confirm what the problem is. He says there is no test for it as it's an old boiler and he can't isolate the part he would need to test. He hasn't looked inside the boiler but perhaps that's normal for this kind of issue.