Hi Bert,
I have not had that problem with mine and not had any of my customers mentioning it either.
How I inflate mine is to open the valve fully, ensuring its been turned anti clock wise to the stop point, when facing the valve. Then place fore finger and index finger either side of the valve with palm of my hand away from the mat, i then push on the pump sack valve, and make sure its fully fitted, then use my left hand to open the pump sac and roll it close to trap the air. Then push it down and squeeze the bag in toward the mat, not straight down.
Can only recall having the pump unplug when I have not had it pushed on far enough.

If thats not the cause and you still have a reciept then might be worth contacting exped directly or your retailer as they are warrentied and seeing if maybe a new pump sack might solve it.

Whem you push the pump sack on it should be a nice tight snug fit and the valve go in about 1.5 to 2cm..

I have recently been considering a poss adapter to allow me to use my electric tyre pump, with a light push fit, to ensure it doesnt damage the mat by over inflation. But thats a work in progress.

For me, the down mats and the comfort foam mats really are the best out there. I cannot recommend them highly enough.

I have some notes I put togather for an expedition group recently about petrol stoves,which I will try to post up shortly too. Might help people with their stove choices.

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safe riding and hope to see you out on the road. .. John..