eDNA Troubleshoot
More likely the H-bridging is shutting down for over current. There is a through hole resistor that program over-current protection for the H-bridge. For low I-trip, the H-bridge will try to drive the motor -> over current -> shutdown -> retry. This infinity loop will cause the H-bridge to heat up really fast.
Solution: Reduce the resistor value (32k ohm = 2A max, 64k ohm = 1A max) to increase allowed current for start up. Reduce or remove and capacitor between motors terminal, only between the motor terminals, not between the terminal and case.
Check direction of diodes between valves terminals. Wrong direction would cause the shift-register to short itself.