Bicycle Mechanics - bent deraileur arm?

I've got a 70s Peugeot road bike (formerly abandoned, a PX-something or other...) with the original Simplex rear deraileur. Seems to work fine when I pedal forwards, but if I roll the wheel backwards, the chain binds in the cages and wants to tear the deraileur off.

What's up-- bent arm? Hanger? Bad adjustment?

clean the derailleur pulleys, then lube them and lube the pivots on your derailleur. next clean and then lube the chain. if this doesn't work, check your freewheel to make sure it is freewheeling; if it's not freewheeling then you need another.

good luck.

The simplex pulleys have a bad habit of losing teeth. If they appear to be the source of the problem, you might want to replace the derailleur. If it's a freewheel problem, you will probably notice the chain tends to roll off the chainrings when you coast. Could be a sticky link; does the chain skip? Also, if the derailleur is not directly under the cog, it could cause this problem; if this is the cause, you'll notice that the chain/derailleur make a lot of noise when pedaling.

My guess is that your deraileur cage is bent between the rollers. When you backpedal, the chain is trying to come off of the rollers. Check the alignment of the two rollers. The bottom one is probably not in line with the top. Just a thought.