not keeping pressure?

The only way you can keep loosing pressure like that is when you have a leak. You cannot keep topping up a sealed system with mains water without loosing it somewhere.
The system should be pressurised to between1.7 & 2bar after all rads have been bled of air. Have a good look round the pump flanges, cylinder conns and rad valves. You must be loosing a couple of cups full by the sound of it.

Cledlyn
November 2008