I have a Bush Hog ZTR, model CZ2561, 5 years old with a total of 550 hours of operation. While mowing this week, I began to notice that the response to the left stick seemed to be weak. By the time I loaded the mower on the trailer to return home, it would barely climb the ramp of the trailer. After unloading, the mower would barely climb the slight incline into the barn. Fluid levels appear to be fine. Filter was replaced at the beginning of summer (about 90-100 hours ago). A friend told me to check for "blocked screens", but couldn't describe where those screens might be accessed. Any suggestions will be appreciated!