Whether you're servicing or replacing a valve, we always recommend to loosen the valve (using the proper valve tool and not pliers) with the tubes inflated, but to finish unscrewing the valve with the tube deflated and whilst holding/trapping the inner part. The reason for this is whilst many tube builders will use something like AlphaSeal 132 or Sikaflex to bond the
back part of the valve onto the tube fabric, not all companies do this. There's also a chance it may come detached when undoing the valve.
If you drop the back of the valve inside the tube, it's possible to relocate it again, but it can be a right faff....especially if you have extra cladding on the tubes.
A second pair of hands if often always helpful when servicing or changing valves.
So to answer the question, yes, there is a chance the back of the valve may drop into the tubes when the valve is unscrewed.
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