Help with solving an uneven z axis calibration  

Help with solving an uneven z axis calibration

Hi

For a while I have noticed I have an uneven z axis calibration, where the rear left quarter of the print bed appears to be lower than the rest. I can see the same effect in calibration print runs, where the horizontal lines thin in the back left quadrant but are fine everywhere else. Here is a picture of a first layer print - top left you can see the slightly more sparse coverage. Its even more apparent during the calibration run where the rear left has the "not squished enough" appearance, whilst the rest of the bed looks great.

I have tried using the bed leveling adjust on the rear and left in the full range from 0 to 50 um, as well as experimenting with lowering the right and front in combination with the back and left being lowered. This just results in the print being horizontally skew in the quadrant opposite the changes. I have run many, many calibration sequences for both XYZ and Z height, and I have adjusted the live z adjust. As stated this results in a good coverage on the rest of the print, but in order to have the back left see a good coverage I almost have to lower the live z too much for the rest of the bed.

I am using standard Prusa PLA that I got with the printer, and the difference in height is consistent with every print, ruling out filament variation. I have even tried physically grabbing the printer to try and manhandle the rear of the printer upwards, though I am leery of removing bolts given the amount of time it took to get the bed to an "all axes are perpendicular" level of calibration. I have changed the nozzle and heat break to rule out clogging. I am running at default pla temps.