Pallet Truck repair

OK not model engineering, but frequenters of this newsgroup have loads of experience !
I have a Lancer Boss 2000Kg pallet truck that has just developed a
fault. It pumps up but then sinks back to ground! Presumably either a piston seal has gone, or possibly the let down valve is leaking - should be a simple repair, but I'm blowed if I can see how to pull it apart. Anyone been there and done it?
( Its a 1985 WH 20/68/120 -NN p/n 7716502 )
AWEM
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Is the release/lower lever mechanism catching anywhere? I've seen that happen before, discovered by the fitter trying to mend it AFTER he tore down the hydraulic pump and rebuilt...
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