I wonder if someone replaced the QD fittings with cheap ones or fittings made for CO2 instead of hpa. The oring seals at the front as has already been mentioned but the QD fitting could have a sloppy lock up due to the metal being pushed away from force causing the seal to not seal I would replace those QD fittings with good ones made for HPA before filling anymore.
Frank,I looked at my guppy last night and do not have the marks on the foster from the balls. I do take the hose off and store it from the tank, so they are not staying in the same position for long. Also, did not see anything that said "foster" on the male.