Shimano Shifter Repair

[cmbicycles]

I have a pile of broken shimano shifters, mostly 9spd vintage. Most upshift fine, but don't downshift w/o having to hold or lightly push the other lever. Anyone have any luck repairing these?
I have degunked and relubed them. I disassembled a couple in hopes of finding something obvious, but it seems not so obvious what would cause it other than weakening spring tension. I enjoy tinkering with this stuff for fun,which is a totally different discussion altogether. I am wired that way... or wierd that way, depending on how you arrange the letters. But any help would be appreciated.