Kayvan- thanks very much for the reply, much appreciate the feedback.
I'll follow up on the link you posted and see what type of cap it is.
I guess the next question to which there seems to be no definitive answer, should it be a vented or non-vented cap?
My car's a late 1971, originally US spec with the flat venting canister on the left side of the trunk.
There was also an occasional fuel leak onto the bumper, and every time I opened the cap after driving there was pressure release as well. I suspect that the pressure build up somehow pushed the cap loose. If the car sat for a day or two I often found the fuel cap came loose, and I had to unlock and re lock the fuel cap prior to driving.
Also replaced the cork seal several times as well as adding a rubber seal to make a tighter fit.
I have a cheap spare plastic locking cap which I had purchased from SLS for €16- just in case I lost the original cap. Which I'm now using until I receive the chrome cap on order.
FWIW the fuel tank is new, new sender unit, new fuel pump, fuel filter and lines I've also cleared the venting lines from the filler neck to the venting canister. So I'm not sure why there's pressure build up in the tank?
Possibly the venting canister is clogged up with old fuel debris from prior to me replacing the fuel tank?
Maybe as an experiment I should drill a small hole into this plastic cap to see if this stops the pressure build up in the fuel tank?
There must be a valid reason for Mercedes having two types of fuel caps, vented and non vented...
I'm confused, any input or ideas?
Thanks & best,
Mike