Exactly the same size as the pump (or differential) cylinder.
Shaped to suit the pump Plunger (the Alderdice Pump has a scalloped seat on the Plunger Block and Spacer)
Permeable (so water can penetrate through the material and therefore lubricate the sliding surface)
With built in lubricant.
Expandable for its own wear.
It is only leather that satisfies all these requirements. Neoprene (which are actually made from nitrile) buckets will last longer, but they tend to cause the windmill to operate under an increased load.