vaillant eco tec pro water pressure keep goes down?

I have vaillant eco tec pro combi boiler fitted in my house i have just noticed that the water pressure of boiler keep dropping down and i just make it up with filling it up, now i noticed some water leakes from the boiler, when i open the cover of boiler there was loads of water i just soaked the water out with the tissues but cant find out where it leaks or is the couse for the water presure keep goes down? pls help, many thanx.