Purging Hydros is a hit & miss sort of job at the best of times.
What most do not bother to note is the engine should be run as slow as possible to prevent the oil frothing .
The workshop where I get my hydros rebuilt has an electric motor set up to bleed hydros and it would be lucky to be doing 100 rpm.
I have needed to resort to things like putting a vaccum line on overlfow tanks to get some to purge while others will purge if left overnight.