Need help with a pulse/counter input circuit on hall effect flow sensor
According to the datasheet, the 1734-IJ has an input current specification of 19.1mA @ 5V input. This means that the hall sensor must be able to provide that much current (which is obviously not the case).

So you need a e.g. an op-amp circuit (google for "voltage follower" or "buffer amplifier") to buffer your hall effect flow sensor output. The outputs of most op-amps are able to deliver more than 19.1mA into a load.