Low pressure in my boiler?

My central heating system occasionally loses pressure and falls below the required water pressure level and locks out, the water is seeping from the drain plug outide, i thought i had it sorted last time but obviously not, the main problem is that after re-pressurising the boiler it can take hours, sometimes a full day before the low pressure light goes off and the boiler fires back up. Any one help please ? the boiler is a petterton puma 80e and is about eight years old. Other than that it works fine.