I think what is happening is that your pump is pumping faster than your buckets can handle an even flow. What you need to do is either replace your pump with a much smaller one, or place a valve on the exit side of the pump and and slow down the amount of water you are sending to the back buckets until they fill up evenly. 100 gph would be more than enough, and you might even have to shut off part of the water flow for even that.

sn