Intermittent or No Brewing

I've got a Keurig K70 coffee maker, and it's been experiencing an intermittent issue that when you press the Brew button, nothing happens. I've tested the unit without a K-cup and intermittently - it will consistently output the correct amount of water.

So, I've got no problem taking stuff apart - and from what I can tell, there's nothing "plugged up" and this is not a de-scaling issue, because when the unit "brews", it brews just fine. If it were plugged or "scaled", the problem wouldn't be intermittent.

From what I can tell, these Keurig machines are pretty simple. They have a small 12V impeller motor that pumps water to the heating tank. Then there's also a 12V mini-air pump, which I believe provides the force that pushes the heated water up into the K-cup holder.

With an alternate power source, I've tested the pump motor, so I know that's working okay. That tells me that the air pump may be the source of the problem. I'm not sure how to test the air pump, and while it sounds like it's running during the brew cycle - I can't tell how well (or even if) it's really working. I can order a replacement air pump for a little over $5, which seems like a suitable investment in a coffee maker that's still probably worth more than $75. Any other suggestions or observations on my diagnostic logic so far? Thanks --Chris