If your Y axis rods are not parallel, it will twist the bed as it moves making leveling next to impossible. A symptom of this is when leveling the bed, 3 corners are right but the 4th corner is wrong. Fix the 4th corner and now the other corners are wrong. And the cycle continues.
You’ll need two rulers or straight edges. They should be of similar height, perfectly straight with both long sides being perfectly parallel. And the edge should be flat so it can stand on the edge. In the pictures below, I added a strip of red tape to the back ruler to increase visibility. I also had to use the lower plate to help keep it standing on edge.
- Raise the extruder out of the way. Take the heated bed off and gently set it aside.
- Set the rulers or straight edges across the Y rods. One as far back as possible, one as far forward as possible. They should be parallel with each other and perpendicular with the rods.
- Line up the tops of the rulers in your vision. It should be easy to tell if the tops of the rulers line up. If they don’t line up, the rods are not parallel.
- Loosen the screws on the caps holding the rods to the frame and raise and/or lower the ends of the rods as necessary to bring the rulers into alignment.
- Reinstall and level the bed.