I just wanted to close the loop on this issue in case anyone was having a similar problem. Turns out the check valve (on the rack side) was stuck and preventing fluid from the supply line flowing in the proper direction into the rack.
I pried the cover off the line, and tried to extract the plastic valve with needle nose pliers. It was stuck fast and would not budge. I ended up taking a hacksaw to the fitting and using a nail punch to get the valve out. It looked a bit deformed and I was confident this was the cause. Long story short, I purchased a line from a breaker, popped it on and the fluid began circulating normally. I'll post a pic of the mangled fitting another day.