In an industrial application you don't see motors in series that often but in agricultural applications, where the flow rate from the tractor (or whatever) is somewhat limited, then it is common for motors to be run in series - it's just as if the oil is in such short supply you have to use it several times over before it's allowed back to tank. What this approach does mean is that the inlet pressure of the downstream motor is felt as the outlet pressure of the upstream motor. Both of the upstream motor ports find themselves at a high(ish) pressure and the upstream motor case connection (if optional) has to be let out to tank separately. It doesn't matter to either motor what the displacement of its "partner" is - if the displacements are different then the speeds will be different for the same flow rate.
You need to check that the motor case drain connections are really being used correctly and that your customer hasn't inadvertently connected them to a place other than tank. Also check that there aren't any isolation valves or non-return valves in the case drain line. If this is a system connected to the HPU/tractor via quick release couplings then check that the drain line quick release couplings are properly matched/mated and that the internal check valves in the couplings are opening properly. If the lines are a little small and the oil is very thick (cold start-up) then just the back pressure could be enough to push out the shaft seals. Is it possible for you to tee in a pressure gauge right next to each of the motor case drain ports? This would let you check the case pressures directly and see if the problem was from something external to the motor.
But if the problem isn't in the execution it might be in the circuit design or the component selection. What make/model of motor are they? The particular sizes make me think of the Sauer Danfoss OMP range but there are lots of similar products. Do the motors have to run in either direction? Do they ever have to stall? What is your system pressure? Do the motors run at high speed with a high inertia load on the shaft? Do the motors have any cross port relief protection? Can you post a circuit diagram?
DOL
