Hi
C1 means that the brake light bulbs are blown
81,26 and 56 are remote input errors. These are probably occuring when the truck faults and restarts, the direction lever still being in either forward or reverse will bring up 81 and 26, not sure what brings up the 56 but it is a similar cause.
51 is a failure of the pump controller contactor coil. Either the power to it or the coil itself. I suspect that the contactor in your truck is dropping out in use which essentially turns of the high power to the pump logic.
15 is a by product of the 51 error code. It is the traction logic safety cut out when it no longer recieves a signal from the pump controller. I suspect that the pump controller supplys the power steering, so for safety reasons when the pump controller fails, the traction controller will not allow drive with no steering
Check the pump contactor and supply to it
Hope this helps