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Brake lights intermittently working on a 2007 Mack MR688


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So I've got a Mack MR tractor that I'm turning into a dump truck.  Working some of the bugs out of it and today I noticed the brake lights are intermittently working. This truck has the brake lights and turn signals separate. When I step on the brake, they flicker.  Sometimes they work, sometimes they don't.  I can hear the relay clicking.  I've got a generic fuse/relay diagram and there's a couple of relays that mention stop.  Looks like many of them are the same and I switched them around with other relays and still no luck.  I put a test light on the terminal/post marked stop and the test light acts just like the brake light.  I'm pretty mechanically inclined and trying to figure out what it is.  I'm thinking it may be the actual switch that's activated by air pressure when you hit the brake pedal.  Have no idea where it's at though.  Any other ideas?

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