PSI gages bouncing from 90 to 135psi

[SquareOne]

When you say it's 'bouncing', is it happening rapidly, or is it actually 'bleeding' from 135-90psi?

I see you mentioned a couple seconds, which could indicate a very large leak if so... I've had the park air brake valve (on the rear frame under the 5th wheel) leak really bad on my Cascadia a few years ago. Only leaked when the brakes released because of the pressure applied. Easy to miss if you're in the cab. Only indication would be the gauges dropping and air constantly building and popping off.