Shown below are two sorts of piston pumps. They both work on the
same principal, namely they are fully positive displacement
pumps. The difference lies in the valve systems used in that the
one, a clack valve system has flat valves, while the other has
ball valves for better sealing. The latter pump is specified for
deeper boreholes and therefor has an extra sealing cup washer
(three vs two on the clack valve). The clack valve system should
not be specified over 100 m depth. A variant of the ball valve
pump is that with certain designs the pump and attendant sealing
washers may be serviced without removing the water pipes and
brass pump barrel. This type of pump is known as a draw plunger
pump and should be considered for boreholes greater than 30 m in
depth.

These pumps are constructed of brass for long life corrosion
resistance.

The piston rod attached to the pump piston should preferably be
made of brass as galvanized steel results in the zinc on the
piston rod "plating" off the steel onto the brass of the pump.
The rod then rusts through much more rapidly.

Provided modern technology windmills are selected most wind
driven water supply schemes fail due to pump failure. This is due
to the fact that the cup washers are often made of leather as
shown in the picture above. After a period of about a year the
leather washers need to be replaced and if this is not done no
water is pumped. The second failure mode is that the leather used
to manufacture the cup washers has differing water absorption
characteristics. As the leather absorbs water it swells often
resulting in the piston locking in the pump barrel. The windmill
then will snap the pump rods connecting the windmill engine to
the pump.

There are two items required to solve this problem. Firstly
modern day pump washers made off synthetic materials such as
neoprene are available however the construction of these to
ensure correct sealing is a well kept secret and only certain
designs of synthetic cup washer are satisfactory. Experience has
shown further that synthetic cup washers have a life three times
longer than leather ones. 
The second design requirement is to incorporate a weak link in
the pump rods connecting the windmill engine to the pump. This is
usually achieved by replacing one steel pump rod length with a
length of wood above ground. This eases the repair function
substantially. This weak link is also useful in protecting the
windmill from failure due to water hammer which is discussed
under hydraulic aspects.

Another type of windmill pump is a siphon pump which is installed
outside of the water source. This consists normally of a standard
clack valve type pump which is braced between the tower base. It
may derive it's water by means of a tailpipe extending into a
shallow well under the pump or by means of a pipe leading to a
river etc. In long supply pipes suction chambers (surge tanks)
must be fitted.

Footvalves should be installed on all borehole pumps except draw
plunger types to act as backup for leaking pump lower valves. 

Strainers are strongly recommended to prevent large particles
from entering the pump and lodging under the valves rendering
them inoperative.