I have a milwaukee flashlight with a sliding head/lens. Problem is that it is sealed so tight to atmosphere that the barrel acts like a piston and either pressure or vacuum developed in the flashlight by sliding the barrel in either direction tends to push or pull the lens/barrel back to 'neutral'. Irritating as hell. Other flashlights do this? Best remedy? Drill a micro hole somewhere for pressure relief? Less watertight, but the focusing feature is all but useless presently.
Spot/Flood sliding lens focus won't stay put.
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Fireclaw18
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This is why most zoomie flashlights are not airtight. Since the action of the sliding bezel changes the internal volume of the light, it is necessary to leave an opening so air pressure can equalize. Otherwise you get the piston action you noticed.
Some air-tight zoomies solve the problem by having a really stiff bezel that must be unscrewed with 2-hands. That can work, but obviously isn't as convenient to operate as the simple one-handed push-pull type.
For your light: No need to drill a hole through the light. Just remove one of the o-rings. For best results, try removing the o-ring between the lens and the front part of the bezel. However, if that one isn't convenient, removing the tailcap o-ring should help greatly. But note that removing an o-ring near the bezel should make zoom cycling more responsive than if you remove a tail o-ring.
Some air-tight zoomies solve the problem by having a really stiff bezel that must be unscrewed with 2-hands. That can work, but obviously isn't as convenient to operate as the simple one-handed push-pull type.
For your light: No need to drill a hole through the light. Just remove one of the o-rings. For best results, try removing the o-ring between the lens and the front part of the bezel. However, if that one isn't convenient, removing the tailcap o-ring should help greatly. But note that removing an o-ring near the bezel should make zoom cycling more responsive than if you remove a tail o-ring.
Put some sort of lube on the outside and slide it back n forth to get it between the o'ring and the head. Probably some kind of rubber friendly grease. See if that works before drilling. May make it worse. 
It's really difficult to get a zommie water tight without the issue you are dealing with.
It's really difficult to get a zommie water tight without the issue you are dealing with.
Fireclaw18
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Put some sort of lube on the outside and slide it back n forth to get it between the o'ring and the head. Probably some kind of rubber friendly grease. See if that works before drilling. May make it worse.
It's really difficult to get a zommie water tight without the issue you are dealing with.
Greasing up the o-rings may help the sliding bezel slide better... but it won't solve the piston action problem. If anything it will make it worse, because lubed o-rings seal airtight better than dry o-rings. And a well-lubed o-ring under the bezel slide will make the bezel easier to move so it will respond even faster to air attempting to equalize.
Most cheaper zoomies solve the bezel action problem by intentionally omitting an o-ring near or at the bezel. Ideally you still want an o-ring directly under the sliding action to give a nice smooth slide. But one of the other bezel o-rings should be removed. Another option is to maybe cut one of the o-rings into a c-ring. Only a very tiny gap is needed so air can equalize.
Doesn't sound like OP cares whether his zoomie is watertight or not. If you want a truly watertight zoomie it's best to get one designed that way from the start with a screw-action bezel that's stiff enough to resist the piston effect.
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Fireclaw18
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I suppose something like this, some form of this relief valve in micro miniature, should have been designed in originally.
Unless that valve is two-way, it won't solve the problem. A one-way valve would work once, but wouldn't allow continued cycling of the zoom mechanism.
Frankly, the problem isn't hard to fix... just pull out an o-ring and you're done.
