Titan oil tank lever valve

[scrumpyjack]

I have a top feed oil tank and I think the lever valve is knackered
It has a red top/latch which locks the valve in the up position, but when I put the lid on the top of the tank it pushes the latch over and the valve closes,cutting the oil supply.
Even if I turn the red top down carefully the plunger slowly closes,cutting off the flow.
How should this valve work