Al-Ko damper

I borrowed a friends Abbey GTS 215 the other week and whilst towing heard, on several occasions, a loud bang on acceleration after braking. I am sure this must mean that a damper has worn on the towing hitch. I have looked for one on the Al-Ko head but it is not apparent. I can push the drawbar back to the stop quite easily? The caravan is a 1996 model if anyone can help. Thanks in anticipation.