Draining down to replace pump and valves

Just an enquiry, I was draining down the Pulsa coil 200 using the drain off which is attached to the pipe work on the pump. It seemed as though it was draining for quite sum time, and wouldnt seem to end. It got the the point i had to remove the lower pump valve with the check valve attached and as i was removing there was water gushing from the joint. Is this due to the system i was working on is based in a large complex with various appartments. Just wondering why this wouldnt drain, I was on draining this for a long period of time and it still wasnt dry. Any suggestions?

PC2000.
2 drain off points
1 on unit at the bottom.
1 on the pump pipe work.
But it may be that the installer has put a drain off on the cold into to PHE or even the DHW out of the PHE.
Anther reason why it can take ages to drain. The drain off washer is stuck and it is draining from just the centre hole of the doc washer.
And if the case can be an hour or longer!
Mike
MGC/H2O
Norfolk