I have a Parker 51 that goes dry on me after sitting for a day.
I've re-built it... the nib is aligned with the large slit on the collector,
the feed is clean, the breather tube has no issues and is clean,
the nib is seated correctly to the hood....
Once is does start writing it writes beautifully. Any clues as to
what might be the culprit here and how to correct the problem
would be appreciated.

What ink are you using Ray....

Griz, just Waterman Fla. blue. Waterman inks are all I use anymore.

Ray,
When your pen sets for a day, is it nib up or horizontal? I've found my 51's respond better to laying horizontal while at rest.

This one is always horizontal.

The inner cap may be loose. Try holding the cap end to your mouth and blowing through it. It air goes through, the caps letting in air thats drying out the nib. If thats the problem than either you can get that inner cap replaced (or do it yourself if you've got the tools) or epoxy it in place, making sure you make an airtight epoxy seal. You'd need to get the jewel, clip and screw out before doing it, of course. The epoxy option might be controversial here, but I'd rather do that then spend the money the other options would cost, plus it has worked fine for me in the past.

Bingo! Tried what you said and air went through.... looked into the cap with a flash light and the
inner cap has a nice crack in it. That's not something I'll attempt to deal with myself.

The inner cap can be replaced. In some cases they can be repaired, but in either case, have to be pulled.

This is one of the reasons why I like to adjust the cap clutches when I repair a pen. When the cap has loosened up, the owners tend to jam the cap on until the section sticks in the inner cap. Stuck caps, cracked inner caps, or marks on the section result.

Manufacturers design "snap on" cap intentionally lettle loose. If the cap is airtight, you create strong negative vacuum everytime you take a cap off sucking ink out of tank. This is accident waiting to happen. Many "snap on cap" designs have a small hole placed behind clip or cap is made intentionally loose so that air can gets in when cap is pulled. You want to be very careful and know how you take a cap off if you make the cap airtight. In theory, if air gets inside the cap and ink keeps evaporating into the air, capillary action should keep ink feeding to the nib until tank become empty. Nib getting dry while resting ususally means that there is a part in the ink flow system that capillary action is not working and that the part beyond that point gets dry while pen not in use. It could simply means that the slit is too wide and capillary is weak there or feed and nib fitting is loose, etc. Please be aware what "making cap airtight" could do.

What you state may be correct for some pens, BUT NOT the Parker 51...

Tom is right, this is not the case with the Parker 51. To release the vacuum created when you pull a cap off, many of the inner caps have a groove cut down the outside of the inner cap, which lets air in from the clip end of the cap. There is supposed to be a good seal between the hood and the edge of the inner cap. A pen can tend to dry out if the edge of the inner cap is damaged, or there is a crack in the inner cap.

Thank you. It is always good to learn things about pens that i am not too familer with