This Dunlop Crybaby GCB-95 wah acted like a volume pedal. Two of the most common faults which could cause this in a crybaby are: broken 4,7uF electrolytic capacitor or inductor. In this case it was the inductor which is a much more costly part than a capacitor (0.40€ vs. 20€). Good thing is these circuits are easy to repair. New inductor in and problem solved!