I haven't had Clarks for awhile but every Clark I've ever worked on has had a flow control valve at the base of the lift cylinder. This should slow lowering to a reasonable speed even if the lift line breaks.
Does it do OK if you lower more slowly? If the load comes down like someone dropping a flower pot from the roof, that's the real problem. A new breather cap wouldn't hurt either, even if the old one seems clear.
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