T500 plastic bits

[Peteren1]

From what I've read the main T500 failure point is the top plastic cap.
I see some have brazed copper caps into place when this part fails which appears to do the job but it seals up the column and would hinder maintenance?
I'm pondering preempting the failure of this part by machining a like for like replacement out of metal.
Brass is out of the question, copper would be nice as well as 304 SS but getting a small junk of either is difficult.
What is the issue, if any, with aluminium in this application?
Cheers
Peter

[The Stig]

I’m just guessing but aluminum will probably oxidize and leach into the grog