Ok so I was having trouble inflating the tyres of my W123 240D.
I have recently asked MB to replace the old ugly rubber/plastic valves for some metal ones (part number 1074000013).
But a couple of days ago when I went to my local BP petrol station I couldn't for the life of me properly fit the air pump nozzle to the valve so instead of inflating the tyres I was deflating them.
I actually had to go to MB where they had no trouble inflating them. They couldn't explain to me why it happened but their theory is that petrol station air pumps are a jack of all trades made to fit everything from bike to truck valves while the one they use is specifically made for regular cars.
When I came back home I tried my ForTwo's valve dust caps on the new W123 metal valves and found they fit perfectly so it does look they're the same size as any other tyre valve.
Scratching my head I asked around and got this explanation from a Portuguese cab driver, apparently these valves are too short so what's happening is that the pin inside the air pump nozzle hits the edge/rim of the valve before getting deep enough to perfectly seal around the valve and inflate the tyres without air escaping.
Mystery solved and so I have placed an order for a Ring RAC900 air compressor on Amazon.
I have also bought a flexible tyre valve extension. This last item cost me 10 quid and should be enough to solve the problem but since my 240D mostly "hibernates" during the winter months it's good to have an air compressor at hand to keep the tyres tip top without going out on the road.