It is almost certainly hydraulics - Try just bleeding it, it often will get you going.
It is also possible that the nuts attaching the pedal to the pushrod have come loose - the Isuzu vibration can do that sort of thing - removing the cover should confirm whether that is the problem. Also could be the master cylinder piston is both leaking on the primary seal, and is stuck in the cylinder.
But the most likely problem is that the slave cylinder seal (or possibly the master cylinder secondary seal) is operating as a one way valve, letting air in.
A mechanical failure of the release fork is possible, but I have never heard of it on this vehicle (common on later ones).
John
John
JDNSW
1986 110 County 3.9 diesel
1970 2a 109 2.25 petrol
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