Ensuring even flow in liquid pumping in distribution circuit under suction

I have a liquid circuit which looks a bit like that:

The system is under suction, and the flow rate through each unit should be about 10ml/min. However, we noticed that unit n and the ones the furthest away from the pump and bottle were not getting enough flow. Any suggestions on how to make sure each unit gets about the same amount of flow? My initial thought was to add a second pump at the end of the line to help the liquid (water) go round the circuit. Another possibility is to separate the load of n units into 2 circuits in parallel, 1 to n/2 and n/2 + 1 to n at the bottle/pump location so that both circuits will have a better chance of seeing the same amount of liquid.

Some precisions:

• there is a restrictor on the return path of each unit to restrict the flow and create a pressure differential to allow the liquid to go round
• the length from unit #1 to unit #n is about 3m, the pump and bottle are located between unit #4 and #5.
• the tubing diameter along that length is 4.8mm ID, and each teed off line in each unit is 1.6mm ID, with the restrictor 0.38mm ID (~60mm length)
• the pump is a diaphragm liquid pump
• there is a vent port with a filter on the bottle lid to allow for the pressure to vent out.
• Totally not my domain but looks like converting from bus architecture to star architecture would help :) – SF. Feb 5 '15 at 11:34