Hello everyone, 
As the title says I have a D8H (46A10231) that won't stay in great at low RPMs. I have tried near everything. I'm only getting around 70 PSI at the pump at idle and jumps to around 150 at high idle, but actuating steering or brakes it spikes to around 300 PSI.  I've replaced the pump (3x). I've had all of the valving apart and cleaned, tried swapping internal valving from a working D8H, I've torn the transmission completely apart and replaced piston rings in the transmission, measured the clutch packs and everything is in spec. I've had the steering valve apart and cleaned, the brake diverter valve apart and cleaned. I've replaced ever o-ring possible. I've removed the suction screen at the bottom of the transmission. I've taken the torque converter inlet and outlet valves apart and cleaned, messed the springs. I've tried blocking off to the steering clutches. So yeah about everything the book and Caterpillar suggests and more. This machine has a K engine in it but I believe an H torque converter. I also found that I'm getting 290 PSI out of the recirculating pump and the book says that should only 12-15 PSI. I hope that's enough info. Anything helps! Thank you for your time. 
(Picture of machine in current condition for your viewing pleasure.) 

I'm confused by your pressure readings. The transmission pump should make 300-350psi in neutral (controlled by the Modulating Relief Valve).
If the pressure drops below 100 psi the safety valve will kick the speed selector into neutral.

The steering clutch and brake system is a separate pump and circuit.
Should make 250psi with clutches released
Maximum 300-350psi with brake activated.

The only spec for 15-19psi is the transmission lube oil pressure which comes at the end of the transmission oil circulating circuit.
Make sure you are using testing specifications for s/n 46A3049 to 46A10724