Not sure if one is available for a Roland or not but there probably is. Look into a sealed ink bag bulk setup. They are more expensive but far more reliable, less messy (zero spills or drips), zero ink contamination, zero ink exposure to the air, not susceptible to level issues, will cause zero pressure issues, etc. Their improvement over the older open tank style (with intermediate float carts or whatever) is orders of magnitude better technology. Considering your stock ink system was designed to be a closed system there are literally a huge amount of potential issues with putting an open system on. Pressure issues being one big problematic bit, your stock cartridges simply collapse the bag inside as ink is used, this causes no pressure differentials in the ink train. If it is having to overcome a float or gravity or a rigid open tank with a closed lid then there are pressure differentials (both negative and positive depending on what is happening) that are not present in the stock setup that can very well cause issues. You also are exposing the ink to open air, that in itself can be problematic since (even if just a little) that is allowing the ink to absorb moisture in the air, some evaporation (curing), etc. which even if just a little is slightly changing the chemistry of the ink. A bulk setup that uses bulk ink bags keeps all the pressures the same as if it were using factory carts, eliminates the need for mechanical floats, and never exposes the ink to air.
Regardless - as Jack mentioned. If your bulk setup is using sub-carts with floats then check to make sure the floats in the black ones are not hanging up. If it is sticking even just the tiniest bit then your pump(s) are trying to overcome that which is causing huge negative pressures in the ink train.
