Lister Domestic Water pump - air chamber bleed screw (?)

I have just acquired a DEP 1 pump with a broken air screw assembly on the top of the air chamber. What does this do? If it is just a bleed screw then can I fit a cheaper 1/8 BSP radiator bleed valve instead?

You can, the idea of the bleed screw is just that, the air chamber works as a "damper" to take the knock or "water hammer" out of the joining pipework caused by the piston, by adding or removing air as needed this works as a "cushion". If the pump is only for rallying then I would just put a 1/8 plug in it and leave it at that as you would not have enough "back pressure" to cause the pipes to "knock".

Many thanks, that solves a pocket problem!