I think aluminum bodies are less prone to leak, but their likelihood to stick is probably the same.

Yours sounds like it is stuck.

There's a lot of information here about servos, but most will say that when they begin to fail, they don't suddenly get better.

I'm on my 3rd servo (this one is aluminum) in 10 years of 6.9 ownership. When this one breaks, I will be getting the electronic replacement from unwired tools.

The manual at the w116.org site has the test procedures and troubleshooting steps. Look at mercedesshop Vintage/helpful links/116.org/handbook