You may have dealt with this already but if not, firstly, don’t put any leak sealer in it. Secondly, did you say topping up every 3 months or so ?
Couple of things to consider, when topping up, under what circumstances is this being done, what is the pressure dropping to before being topped up what is it being topped up to? Is the system cold when being topped up (very important that one).
In a perfect world, we would set set pressure when system cold to around 1.2 - 1.5 bar (depending on boiler), when pressure drops to below 0.7 - 0.8 bar, would top it up again, this depending on size of system could be around very 3 to 4 months. Sealed systems naturally lose pressure over time through air/hydrogen being produce, then venting through auto air vent, water evaporates as well. Chances are this is normal operation.
hope that is of some use
BG engineer