Totally agree. I have soaked, washed and scrubbed parts that when put into an ultrasonic cleaner just seem to leak ink. There seems to be a real affinity between ultrasonic cleaners and dried ink. If you want to really clean your pen - you have to use an ultrasonic with HOT water and a bit of pen-cleaner (Koh-I-Noor is my favorite - what’s yours?).
Many older pens can be successfully restored by just taking it apart for a deep cleaning. Parker 51’s with aerometric fillers are like that. The sac is almost always fine, just completely clogged with old, dried ink. A couple of runs through the ultrasonic cleaner; and when put back together they work like new.