I recently purchased a 76 450sl that had been setting for many years. Question....is the air pump pulley supposed to be able to be turned by hand if it doesn't have the belt on it? I can't seem to turn mine by hand but I don't know if that's normal or not.
It should turn.
However, as Aussie says, it's not needed for running (I believe a valve opens and it pumps air into the motor for a few minutes when cold - to help reduce emissions). Depending on the local regulations/emission tests you may need it to work.
Mine was also seized, but where I'm located classic cars aren't required to take an emissions test so I have removed it. Once you've done that you will need to:
a) Plug the threaded hole on the timing change tensioner where air is injected into the block (you'll need an M22 x 1.5 plug) - I've bought this
, but haven't yet fitted, so can't be certain it's perfect.
b) Find two bolts (I think they were M8) to replace those that originally hold the bracket in place (otherwise the timing cover will leak oil...). They need to be slightly shorter than the originals.
c) Remove the vacuum line that leads from the pump to the blue vacuum switch in the inlet manifold near the front, and replace with a cap (I've not done this yet - assume I'll use a simple rubber or silicon cap)
I've not finished the job yet (I've removed the under hood catalytic converters as well, while replacing my front subframe and rebuilding the front suspension on my '76)