Hello,
I have a Raleigh Hybrid that is about 25 years old. Great bike but recently the rear derailer is causing an issue. When I quit peddling, the rear derailer will turn upwards, the chain will get so much slack that it almost rubs the ground. If I start peddling the derailer will get back to the correct positon and the cahin works as it should. I have lubed both chain and derailer. Any ideas? Thanks

You might try posting your question in the Bicycle Mechanics subforum.

I would guess that your rear derailleur's behavior is a symptom, not a cause. Perhaps you have a malfunctioning freehub or freewheel. If you have the latter, given the limited supply of freewheels, it may be time to switch to a wheel with a freehub and cassette cogs. Unless you have a fabulously expensive rim--doubtful--freehub service probably doesn't make sense, in which case you could move the cassette (if it's not showing signs of wear) to a replacement wheel.

Place the bike in a workstand or elevate the rear wheel, then spin the wheel. The chain, cogs, and crank should not move. If they do, you would appear to be confronting a freehub/freewheel problem.

You could try removing and cleaning the freewheel. RJ the Bike Guy has a good video showing how it's done; I'd post a link but I lack the required 10 posts.

This is my thought as well. It sounds like the freewheel or cassette continues to rotate forward when you stop pedaling, which will create a lot of slack in the upper run of the chain and I imagine it should pull the derailleur cage forward as it tries to suck all that chain through.

Be careful while disassembling the parts.