One Slow track and slow turntable on Hyundai 210-7 LC

Hi all, First time poster. I have a Hyundai 210-7 LC 22.5T 1999 Excavator with about 5000 hours on the clock. I was using it at full rev a couple days ago and all of a sudden lost power to one track and the turntable. They are weak and sluggish. Changed thr hydraulic oil as it was due time anyway, and had a good look over, but cannot see any leaks anywhere. Suspected faulty pressure relief vales or something like that. New oil and everything else is strong as ever, just sluggish left track (10% power) and sluggish swivel, turntable is about 20% power. Has anyone got anywhere I could start here? Am experienced with tractors, but this is my first digger. Cheers.