Tires Not Taking in Air But Will Let Air Out Have tried a number of times to inflate mountain bike tires with schrader valves using a pump but it never inflates.  Tires already inflated but feel a little low on air, so I go to pump them and the valves are not accepting air, just feels like I'm compressing the air in the pump.  It doesn't appear that the valve is blocked or anything because when I first put the pump on the valve it lets air out.

What is going wrong?
You may simply not pushing the pump onto the value body enough, and the internal valve is not being opened. 

Most pumps have a plastic insert behind the valve hole that is reversible for Schrader or Presta value use.



[Schrader left Presta right.]

This insert may be the wrong way around, or (as has often happened to me) the small peg that opens the Schrader valve has been bent so it no longer pushes on the valve.