Secondary Air Tank Warning Light

[jefflbi]

On my last trip I noticed the secondary air tank lettering above the gauge was intermittently lighting up. The gauge was still reading full pressure and steady, so I really wasn't concerned. But on my way home I contacted a mobile resource tech who I've used before and he told me to check for a "desicant" filter and replace, or a psi switch. In the last month I've had some other mobile techs tell me I should open the drain plug in the air tank, that there is most likely water in the tank causing the warning light to go on and off............. so I do plan on checking for a drain plug later today. But I was wondering about a desicant filter, and if I had one where it would be located.