What we do know is the right side drive motor went down and you replaced it. Like Mike stated, the proper procedure after a motor failure is to flush the system, the most thorough would be disassembly of all components, drive motors and drive pumps, to remove any debris from failed motor. In many cases this is either not practical or is cost prohibitive to the user so they opt for simply replacing failed motor. The danger can be the debris generated by the failed motor can damage or destroy the rotating group of the pump that drives that motor. It's a roll of dice, no way to predict if it will destroy the pump or not, but it can happen.
Here's a simplified diagram of how your system works. The reservoir feeds oil to the implement pump (8). Oil leaves the implement pump (red line) and goes to the loader control valve. Assume you're not working any loader functions, then oil goes straight through the control valve.
Oil leaves the loader control valve and becomes the charge circuit (orange lines). Prior to getting to the hyd filter, the brake circuit (green lines) Tee's off of the charge line to operate the brake circuit.
The charge circuit goes through the filter, then goes to charge (feed) the drive pumps with oil. Pressure on the charge circuit is maintained at 245-255 psi by the charge pressure regulating valve (6).
Charge pressure feeds oil to the drive circuit/s (purple lines). The drive circuits are closed loop, from pump to motor, back to pump, back to motor, etc, the charge circuit simply keeps the drive circuit/s filled with oil as it's lost, either by case drain leakage, popping open a drive relief valve, or oil loss by operation of the drive flushing valves.
Oil lost by leakage at the drive pumps go through the pump case drain lines (blue lines) and goes right back to the implement pump to travel back through the circuits.
A key symptom is the loss of charge pressure when operating the right drive. You don't loose charge pressure when using the left drive, only the right. This eliminates a problem with the pressure regulating valve as well as a problem with the brake circuit, it indicates you have a problem only with the right side drive circuit.
Could it be the motor failing? Not likely, if the motor failed and had excessive case drain leakage it wouldn't necessarily make charge pressure drop. And even if it did have catastrophic failure that made charge pressure drop, it would struggle to drive at all on that side. And you'd also notice other symptoms with drive problem on that side, noises, something you feel in the drive, maybe things you noticed last time that drive motor went down.
What can happen to cause charge pressure to drop as you describe. If you have a bad rotating group on a pump, you get excessive leakage between mating surface of piston housing and valve plate. It may push enough oil to work a motor under a light load, but under hard working load demand, the leakage at drive plate is too great. You now have oil hemorrhaging at the drive pump, oil is dumping out the case drain of pump. Since oil is being lost at this rate, the charge circuit is attempting to keep circuit filled, but the oil loss is greater than what the charge circuit can supply so charge pressure drops, until you let go of the right side drive, pump recovers (somewhat), charge pressure stabilizes. Of course there's no way I can say with 100% accuracy that this is your problem, only suggesting that the symptoms indicate this is your problem.
I hope this all makes sense.
