1996 E300 Injection Pump Help Needed

[oilslick]

The car sat for two years with the hard metal lines removed. The system primed easily enough, but line #3 has a terrible amount of air coming through it even after the engine has run for a good while. When the fitting is cracked open there's an enormous quantity of foam spewing from the line. None of the other cylinders exhibit this behavior. It seems this would be an internal issue with the pump. Any suggestions?