I have a Honda Accord '95 4 cylinder V tech engine. It has ABS system. Once in while only, when driving at low speed and brake depressed at stop sign, I hear a loud 'cluck' sound coming from the front. Sometimes it depends on how I put my foot on the brake pad pushed slowly straight forward or pushing from the top (strange, i am not exactly sure about footing angle)

Do you think this is something to do with brake caliper or ABS system or vacuum line or suspension (sway bar, bushing etc.)

 

Since it depends on the position of your foot, the first thing I'd do is check to rule out strange stuff at the actual pedal, it's hinge, or the clevis attachment to the MC pushrod.