Hi there,
This is Bogdan from Romania.
I'm a mechanical engineer, designer, modeler and inventor with over 15y experience in designing, modeling and effective manufacturing of different products, devices and systems which spans across multiple industries.
For the application you've described the only viable solution is to have a modular positive displacement pump battery/system.
Each pump will fill the bucket with the required quantity, measured by the number of revolutions (if the pump give 0.5l per revolution, will be necessary to have 31 revolutions to fill the bucket).
Another solution but which is more costly and complex is to have a larger pump and a measurement system which to control the differential pressure for each pipe and to adjust the command valves. (personally i wouldn't consider this).
I have several patent applications for pumps (two of these patents refers to peristaltic pumps - which are positive displacement type) and I could design your system to work flawless and also to allow you minimum investment.
The design could include the triggering mechanism, and could be made to comply with different constrains (time required to fill the buckets, the way filling is triggered, when the skid to leave the filling station, etc).
In the first place I'll make a primal model. On this model, I will adjust/modify/change different features as you'll require, until the desired outcome is reached.
I'll deliver you the 3D model and execution drawings.