After using a Parker Vector for years (and a Waterman Hemisphere as my "good" pen), I dove into ebay over the Holidays and ended up with a few vintage parkers. My new good pen is a late "51" special (with the gold nib) that was apparently uninked and writes wonderfully. I also picked up a 45 and "21" Super cheap. The 45 was easy to clean and writes well (although with a very fine XF point). The 21, which I hoped to be my everyday pen, is giving me the most trouble.

It was very clean cosmetically, with the original $5.00 price sticker still attached. The ink bladder was very black -- I think it was filled maybe once or twice, then sat dormant for around 50 years. After several good soakings, the bladder was pretty transparent, and no color was coming out of the pen. I put in some Watermans blue-black and gave it a try. At first I thought all was well, but it seems to write a little dry, particularly on the upstrokes. Since it is basically the same design as the 51 special, I thought it should write similarly. It does okay with more pressure, which I think must be opening up the nib and allowing more flow.

Is this still a cleaning issue, or does the nib need some tweaking? I haven't been able to get the hood off yet, even with a little soaking in hot water. When I first cleaned the pen, I got several flakes of old ink out of it, not just dissolved ink, so I suspect there could still be crud in the collector and feed. Any suggestions for getting the hood off without damaging the pen?

Just because the sac is more transparent does not mean the collector is fully cleaned... if the collector still has a bunch of dried ink in it you will not get proper ink flow to the nib.
I would first suggest flushing with a 10% ammonia solution and then letting the pen sit in the solution with the pen filled with it and the hood submerged overnight..
Flush again with the solution and you will probably find a bunch more ink coming out...
Follow this with a clear water rinse and then wrap the nib in paper towel and set it nib down to allow the paper towel to wick away any water remaining in the collector before filling...

Just because the sac is more transparent does not mean the collector is fully cleaned... if the collector still has a bunch of dried ink in it you will not get proper ink flow to the nib.
I would first suggest flushing with a 10% ammonia solution and then letting the pen sit in the solution with the pen filled with it and the hood submerged overnight..
Flush again with the solution and you will probably find a bunch more ink coming out...
Follow this with a clear water rinse and then wrap the nib in paper towel and set it nib down to allow the paper towel to wick away any water remaining in the collector before filling...

Thanks for the suggestions. I should have mentioned in the original post that I have already soaked the pen in an ammonia solution overnight a couple of times. I don't get any more color out of the pen, although sometimes I see a small fleck in the solution after cleaning. That's why I'm beginning to look deeper into cleaning with the hood off or perhaps other solutions to the slow flow.

YUP, sound like it is time to take off the hood and give the collector, feed and nib a good cleaning.
If you have an ultrasonic cleaner that would be great... you might need some thin brass sheet stock to clean out the spaces in the collector if soaking does not do it...