ASHP tripping on lp during defrost.

Hi all

Been to look at a 90kw air source heat pump which is tripping on lp on 1 circuit. (Unit has 3 separate circuits). I went to it today and set it away, it runs happily on heating, however when it goes onto a defrost suction pressure drops and then ends up way into a vacuum where it eventually trips on the lp switch.

The unit is on R407C, it has no name on it (basically looks like a water chiller), it was installed by a company who as far as I can gather are no longer with us.

As far as I can see there must be something causing a restriction during the defrost cycle and starving the compressor of refrigerant. The defrost performance is poor when compared to the other circuits - very little heat in evaporator, there is frost leaving the defrost expansion valve (during defrost) and disappearing into the condenser but no where to get the gauges onto on this side.

Other than 4 way valve and (defrost) expansion valve there is a limited number of options, I am thinking the defrost expansion valve must be at fault, anyone got any ideas or advice?

Thanks