I have 753 and just recently pulled these 2 brass filters; one that comes from the hyd. Oil tank and the other from a line that appears to come off the hyd. Pump and goes somewhere to front. I just cleaned them with diesel and blew them out with air at about 90 psi. The one from the oil tank was a bright color but the one from the line was darker in color does that mean anything? How do you know when to replace them or do you ever need to be replaced? The unit has about 1500 hours and is a 1999 model.