HC-SR04 desoldered not producing readings, or producing wild erratic readings at distances greater than 5cm

I wanted to create an ultrasonic flow sensor to find the direction of flow of water in a pipe. To do this, I planned to make a transit-time sensor by comparing the duration taken for sound to travel diagonally towards the direction of flow and against the direction of flow. i desoldered a HC-SR04 sensor, and attached the transmitter and receiver back to the sensor board using wires. While the transmitter and receiver do still produce accurate readings when placed opposite to each other with a spacing of 3-4cm, they almost immediately stop producing readings when they are separated by 5cm or more, which puzzles me as I read that I can expect a range of up to 3m with the original HC-SR04 sensor. Help would be very much appreciated. Thanks!

Range when on leads:
I suspect the lead wires being as long as you show adversely effects the readings.

Functional Goal:

You cannot measure flow or directing or anything with an ultrasonic sensor designed for air in a liquid application.

To do what you need you would have to bond the sensors directly to the pipe AND the frequency will have to be much much higher. So basically a sensor designed to use in a liquid.