On March 7th I had a post describing an air leak problem I was having between the top of the hydroblock and the inlet of the deaeration pump. The final solution to this problem was to replace the hydroblock assembly. I had tried two used hydroblocks we had at our facility. Unfortunately they had the same problem.
Once the new hydroblock was installed, there was no more air leak.

Possibly could have replaced the seal on those hydroblocks. Would have been a lot cheaper!!

I did try replacing the hydroblock seal first. It made no difference. Fresenius suggested we replace the entire hydroblock assembly.