I've changed the coolant on my '91 300-SEL about every 2 years since I bought it in 1997.

On a couple of occasions, after changing the coolant, the upper hose would collapse while cooling down overnight.

I have done a search on this problem and am aware of the so called faulty pressure cap.

My question is why do these caps go bad from seemingly nothing more than the removal required to drain/refill the system?

The bleeding process I use works well - no overheating - air seems to purge. A 20 or so mile drive afterwards goes well - once again - no overheating.

I can remove the cap for the occasional topping up without incident. This problem only occurs when the coolant is changed. If the system had air which I suspect some will point to, the car would overheat and I would likely have a disfunctional heater. This doesn't happen in my case.

Thoughts appreciated.

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Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"