You can tell this by holding your hand loosely over the end of
the hose. If you find that it draws the water in and then forces it out
again, the trouble is with the first check valve. There is something
under it which prevents its shutting down. If, however, you find that
there is no suction at the end of hose examine the second check. If
there should be something under it, it would prevent the pump working,
because the pump forces the water through it; and, as the plunger starts
back, if the check fails to hold, the water flows back and fills the
pump barrel again and there would be no suction.
The trouble may, however, be in the hot water check, and it can always
be told whether it is in the second check or hot water check by opening
the little drain cock. If the water which goes out through it is cold,
the trouble is in the second check; but, if hot water and steam are
blown out through this little drain cock, the trouble is in the hot
water check, or the one next to the boiler. This check must never be
tampered with without first turning the stop cock between this check and
the boiler.
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