charge pressure on t 250 bobcat

i am helping on a t 250 wen are losing charge pressure. when we cap the line coming to the case drain filter, we have good charge pressure. i suspect it is in the right side final drive it was re built about 3o hrs ago there are two large lines and two small lines. we are capping what i would call the return line . like i said it is going to the case drain filter much thanks in advance