The PD carb internals are metal, the only rubber parts are in the Idle mixture screw circuit and the bowl seal (orings).
Is there a reason why you need to replace the metal parts? Corrosion, or mad man wielding implements of destruction? If the stock parts aren't physically damaged, reuse them, the same way you would reuse a cylinder block, wrist pin, or camshaft.
I just want some knowledge on whether the carbs were good or if they were just thrown on for emissions
Well, they were introduced for emissions reasons, in that they provide much better control of fuel metering. It was a baby step on the way toward fuel injection and computer controls for more modern machines.
You seem to imply some negative connotations with the PD carbs, and I have to disagree. They were a refinement more closely tailored to the engine needs with the provided exhaust and met import emissions standards.
The 77-78 Cb550s with PD carbs got 5-10 MPG better than previous years and had the same power. What exactly is there to complain about?
As for parts, recall that the PD's were the least populous carb for the 500/550 line. Not too surprising that the internet tends to ignore their existence, as there is minimal sales revenue expected. But, that doesn't mean they don't have the parts, you may have to call some places to determine if parts are available. Although, I expect major cast parts like entire carb bowls will have to come from a donor carb set. But, that's likely true for the earlier carbs, as well.
Cheers,
