Thank you for your input, I will check those items and post the results

On the Doosan 9-series with the electric park brake there is no cable or star-wheel adjustment like the old mechanical handbrakes - holding force comes from a spring-applied pack that the solenoid valve releases hydraulically. Since you can feel it engage and the foot brakes work fine, I would check two things: first, verify the park-brake solenoid is getting full system voltage under load, because a weak coil can let the spool drift and partially release the spring pack; second, inspect the spring-applied discs for wear or oil contamination, which will kill holding torque on any slope. Also re-check the brake release spool setting after the valve swap - if the new valve seats differently, release pressure may be creeping above spec and partially defeating the springs. A pressure gauge on the park-brake release port while cycling the switch will tell you quickly whether the fault is electrical or mechanical.