I am currently attempting to deal with an issue with releasing a FlowDocument resources. I am loading an rtf file and putting it into a FlowDocument with TextRange.Load. I n
I tried to reproduce your problem, but it doesn't happen on my machine.
Task Manager shows working set size, which isn't an accurate representation of a program's memory usage. Try using perfmon instead.
Now repeat the experiment and check if that counter keeps going up. If it doesn't, it means you aren't leaking any managed memory.