Case 15 maxi track motor problem

Hi All

I recently replaced the face seal on the track motor. All was fine for a few hours, then the casing plate ballooned out and cracked.
The motor is being pressurised by the hydraulic fluid. There is no case drain on the motor, just 2 lines.

Does anyone know how the motor is getting pressurised, and how do I fix it?

Many thanks in advance

H