First, no leak should be considered negligible.  Many small leaks add
up to one big leak.  There is no allowance for errors in a restoration.
In addition, running an unrestored pump with the wrong motor at a
higher speed probably has done considerable damage to the old pump
cloth.

It is also possible that the adjusting screw for the knife valve has
been turned so far that the knife valve has reversed itself, and is not
working as intended.

Bruce Clark