My 3cyl diesal ford 4000 has developed a problem with the lift, if I'm doing any thing heavy, EG a linkbox of stones she struggles to keep the lift up you really need to rev her hard to lift it up, I've recently done all the o rings, new pump and changed the filters, everything else works fine good hydraulic pressure on the spools, tipping trailer etc, it's just when you do heavy work, I'm thinking it's the hydraulic control valve? Oil leaking past the piston maybe? Does any one have any suggestions for me, many thanks