No Engine Braking In Manual 1st Gear
WARNING: This page does not describe the selected car, but rather 13 other vehicles, including the 2003 Mazda B4000, 2003 Mazda B3000, 2003 Mazda B2300, 2002 Mazda B4000, and 2002 Mazda B3000. However, it is still accessible from the selected car via links, so may be relevant.
Check the following: improper fluid level, shift linkage, high or low line pressure, valve body (manual low valve, check ball "E" not seating or damaged), coast clutch assembly damaged, low-reverse band or servo damaged, or rear one-way clutch assembly damaged.