That red capped valve isn't the rebound adjuster. It's the SPV (stable platform valve) it sets the level of COMPRESSION platform damping. and once it's set to the preferred level, in a range usually between 75 (light) and 200psi (firm platform) you don't need to adjust it. It acts as a kind of automatic Propedal, lockout, platform that works in the low speed compresion range to allow the bike to pedal well, then become active on bumps. It basically sets the sensitivity of the platform in the given range, dictating the size of bump that'll activate the shock. The blue dial controls rebound on Manitou shocks.
If you release air from the red valve, your shock will run "faster" but not beacuse it's adjusted the rebound, it because it's removed the stability of the oil by effectively removing the IFP (internal floating piston) that supports the oil on a cushion of air that acts as a spring when the damper can't cope and it "spikes". You're effectively turning off a ride tuning feature and spoiling your shock's performance, as well as removing a safety feature, and you could damage the shock by doing so.
Start by setting the SPV pressure to 125psi, ride a trail you know well, and tune it from there. go up or down in 10psi increments until you get close to a good feel, then drop to 5psi changes to fine tune it.
WTTF and HTH.
That was useful! We've just bought an Orange Patriot frame from fleabay and it has the same shock mentioned. I was going to look up the settings etc........no need now. Thanks Kev