I have a 40+ year old Rainbow vacuum that's always worked pretty well but seems to have less suction lately. The hose is duct-taped to the end that plugs into the water canister and perhaps that connection is getting aged, or I fear I've done damage by using it to clean up too much after sheetrock work. Before I spend a pile on another hose I'd like to know I haven't done permanent damage, and I don't know of any Rainbow repairers in this area. I've cleaned what I can clean without taking apart the works, which I figure I would screw up. Any suggestions?

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"I fear I've done damage by using it to clean up too much after sheetrock work"
Yes your fears are certainly justified. I've actually done that! The dust is so fine it is drawn right through the primary filters and gets into the motor where it wreaks havoc on the motor especially the commutator and brushes.
Does it look like this one?
Does the motor sound different or funny?
The sheetrock dust can also clog up the filters.
Test the vacuum with the hose disconnected and place your hand flat over the opening to see how strong the suction is. If it's weak remove the filter protecting the motor and test again. It actually has a HEPA filter:
Remove that filter and test again. Inspect the motor while you're in there. White powdery dust?
If it's stronger after you remove that filter replace it.
It looks a lot like that but pretty sure mine is a D series, not E, no HEPA filter. There's a grey (maybe once white) cup with slanted slots that;s screwed to the bottom of the motor that may constitute the filter... I clean it with a toothbrush. I do think the suction is less than it used to be at the hose port site.
So the problem has to be above in the power unit. You can bring it to me here in Beaverton or see me at the fair on June 12.
"screwed to the bottom of the motor that may constitute the filter"
Is this the model that's bagless? Can you see the brushes and commutator? If the wallboard dust damaged them there would have been a lot of arcing!
"I do think the suction is less"
So there's some problem with the air flow path. That's the only filter incoming to the motor? Is there an exit filter?