I should have read the manual more carefully. It does provide instructions on how to clean the filter. I'm adding to this thread so it might help someone with the same tractor in the future.
Hopefully this could solve the problem...
I recently got a used massey ferguson 1433 HST. Everything appears to work just fine as long as I'm operating on flat ground. At about 10 to 15% grade, the tractor would not go up unless it's on 4 wheel drive and low HST setting. On medium, it will crawl very slowly until the engine stalls...