Sticky / broken floor pump head? I have a Nashbar floor pump with an automatic Presta/Schrader head. The valve holes are on one side of the head and the air is supposed to switch to the hole with the valve inserted. It used to work great but now it won't switch, i.e. it blows air to the side that's not attached.

I tried the normal stuff, banging, wiggling the head etc. Is it dirt inside the head? Worn parts? Has anyone tried to take one of these apart? WD40? Or should I just get a new pump? It works great otherwise; when it's selected to the right hole (sometimes Schrader, sometimes Presta), but 90% of the time it's on the side I don't want! And because of my budget it was one of my "major" purchases!
Most likely a gasket/o-ring blew or some debris is caught up in the internals. I wouldn't use WD-40 as the light kerosene can deteriorate o-rings or some plastics. I'd try with some canned air / compressed air. 

If your pump head is well designed, you can take it apart and rub some silicone lube on the o-rings and mating surfaces to help restore the seal. If you don't have silicone lube, look for a grease that is o-ring/plastic friendly (WD-40 and vaseline are NOT o-ring friendly). In the image below (of a portable not stand pump) the o-rings and gaskets/plunger are the two circled objects in the very middle. 



Note 1:You have to BE VERY CAREFUL of both the order and direction of parts in a pump head, flip them around and the pump head won't work or will work poorly. Taking photos of the pump head while you take it apart is highly recommended.

Note 2: Your pump will be more complex because it's dual-head and designed for one side (presta/schrader) to be automatically closed when the other side (schrader/presta) is in use. 

If this doesn't work, it might be easier / faster / cheaper to just replace it. Do a search on amazon (or your LBS) for "replacement bicycle pump head" or "bicycle pump head upgrade."