Chris Hogg wrote:

(I have two Hozelock 5l sprayers, one for
killing vegetation, the other for combating pests and diseases), but I
often wonder why they do need to be re-pumped quite so often.
Obviously, as the container empties and the empty space increases, the
pressure will drop (P1V1/T1 = P2V2/T2 and all that)

I have the 5l one too (quite old so may vary) obviously if you fill it
to max, then pump it up it's not long before it needs pumping up some
more as the airspace has e.g. doubled for less than 1 litre of liquid
sprayed, but if you only fill 3-5 litres, I find one pumping until the
relief valve gives will empty it all.