I keep having to top up the water in my system as the pressure drops cutting off the boiler. I have found that water is coming out of pipe outside which leaves the boiler by placing a clear plastic bag over it to collect the water (thus establishing I did not have a leak in my pipes under the floor).
Is there a quick fix, part i need to buy or an adjustment I need to make. I fixed my boiler the last time when the secondary thermistor went faulty leaving me with no hot water.

Alan Simpson, October 2008

Your expansion vessel is deflated and needs pumping. Drain boiler and keep the drain valve open and pump expansion vessel (using a foot pump) pump approx 10 pounds of pressure. You may have to replace and/or clean your safety valve.

I. Sharma, October 2008