Carrier J16 Absorption Chiller

Hello, I have recently tried to take a solution sample from a Carrier J16 absorption chiller. Under normal conditions, whilst running, the solution pump should be at positive pressure making it easy to gather the sample. However, when I tried recently the discharge of the pump was on a vacuum. Not having worked on this chiller before I tried reversing the pump rotation in case that was the problem - but no joy. Any ideas? Or will I have to fit a vacuum sampling rig via a vac pump and a flask?