1993 Silverado 2500 Clutch Failure...

This unit uses a hydraulic clutch mech. Truck has 248k miles, current clutch has about 120k miles on it. It's a diesel 5.7 Turbo.
The clutch still grabs just fine, but just out of the blue you could not
disengauge the clutch. Pedal is firm and all, but it's not disengauging completely.
Is there any common fault to look at, or should I just replace the master cylinder? Any simple tests to narrow it down?
Thanks, -Steve
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