Bicycle Mechanics - Fatigued Rear Derailleur

I have an early 80's dura-ace derailleur that I have on a road-bike I built up the past summer.

It shifts really well, however I noticed when I shift down to a lower gear on the rear cassette, the top jockey wheel start to ride the cog. However this doesn't happen when I shift up to the same cog from a lower gear.

This derailleur doesn't have a b-screw to adjust to give me more clearance either. Is this a case of a fatigued mechanism? Can this possibly be remedied by shortening the chain by one link?

This derailleur may set the angle based using an upper and lower spring. The balance between the springs is what's key. There should be an adjustment for one of the springs so you can set the balance needed. These derailleurs are sensitive to dirt or dried grease at the upper or lower pivots because friction can affect the balance or the derailleur's freedom to move to the balance point.

il go with clean and light oil too/..

This derailleur may set the angle based using an upper and lower spring. The balance between the springs is what's key. There should be an adjustment for one of the springs so you can set the balance needed. These derailleurs are sensitive to dirt or dried grease at the upper or lower pivots because friction can affect the balance or the derailleur's freedom to move to the balance point.
Are you referring to the b-screw adjustment? This RD doesn't have that screw adjuster.

I should also note i am using friction shifters as well, if that matters. What is the best/easiest way would I go about cleaning the derailleur springs? Does this require disassembling the mech?

adjust the chain length, you may lose the high end if the # of cassette teeth are beyond the capacity of your derailleur.

Thanks electrik, I think I may try this first. The max teeth for this RD is 26 and I'm pretty sure my biggest cog is smaller than that.

Disassembling the RD for cleaning seems a bit tedious and if shortening the chain doesn't work, that would definitely have to be a rainy saturday afternoon activity...

I know there isn't the classic B-screw adjustment or a fixed stop position for the upper pivot. Look near the lower pivot and see if there's a spring adjustment there. Possibly it uses multiple holes in the cage and you adjust the lower spring that way (common in RDs decades ago).

However the fact that it finds a different balance point, depending on which direction you come to a sprocket from, suggests that it's a friction (dirt, dried grease) issue, more than a spring tension problem.

The balance point is also determined by chain length, but that's trickier because you have 2 chainrings up front, and so are dealing with 2 effective lengths.

Behaving differently shifting up or down leads me to believe that friction is the culprit here. If you don't want to dismantle the derailleur to clean and lubricate it properly you might try flushing it out with brake cleaner and after it is dry giving is a thorough lubrication with some light oil. Work the mechanism around while you spray the cleaner to try and work out any grit.

Shortening the chain may help if it is too long to begin with, but be cautious not to shorten it beyond where it will not reach around the big-big cog-chainring combination. Shifting into that combo with a too-short chain is inviting disaster.

A cleaning and lubrication is the most conservative course of action and will certainly do no harm.

+1 on the cleaning. I can't recall if the old Dura Ace had two holes for the lower pulley assembly pivot spring. But if it does, and someone tried to service the RD in the past, then it may be in the wrong hole and that can throw off tension or cause the pivot spring to sometimes bind inside. You may need to remove that assembly and disassemble to see if there's a washer that's also not correctly in place on that pulley arm assembly. That can also cause binding and unequal behaviour on the up vs. down shifts.

I have an early 80's dura-ace derailleur that I have on a road-bike I built up the past summer.

It shifts really well, however I noticed when I shift down to a lower gear on the rear cassette, the top jockey wheel start to ride the cog. However this doesn't happen when I shift up to the same cog from a lower gear.

This derailleur doesn't have a b-screw to adjust to give me more clearance either. Is this a case of a fatigued mechanism? Can this possibly be remedied by shortening the chain by one link?

I may be misunderstanding the problem, but wouldn't using a shorter chain make the problem worse? It seems you'd need a slightly longer chain. Another possibility is that your rear cluster exceeds the RD's "coverage."