Don't know why no one never said thanks for this, but I suppose it's never to late!
I suspect a dirty/jammed pressure relief valve on my pump, but it looks it can only be accessed with the pump out:-(. I'm able to see the nut, but it looks to be >20mm and no way to get a tool on it.
2003 GMC Sierra 2500HD SLE EC; 123k; Allison 1000; 8' bed; AFE/CAI; MBRP 4" Exhaust; Side Tint, 33% VLT, rear, Limo; Smittybilt Stainless Side Nerf Bars; Sprayed-in Bed Liner; MadJack Fuel Filter Access Door; Regular UOA, Blackstone Labs; EGT/Boost Gauges; Dual Chrome Dipsticks, JC Whitney; Does anybody ever read this s%#t?; Injectors under warranty @ 70k