Rear brake issue M1028

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With the annual check of my M1028 it showed the problem with the rear brake.

Problem: driver side has approx 25% brake pressure and the passenger side works perfect.

I can't seem to get more brake pressure.

Already done in trying to solve the problem:
Load valve sensor delete and replaced with a manual version.
New brake hose
New brake lines on the rear axle ( both sides)
New brake cilinders (both sides)
New brake pads (driver side)
New mounting kit for the pads.
Switched the drums from left to right.
New handbrake cables

Between replacing the parts it has been on the brake test bench to see the difference.
Only there is no difference while all the parts have been replaced.

I'm sure there is no air in the lines.
I have checked it muliple times.
And both sides have been adjusted in the same way. Passenger side is ok everytime.
Driversside keeps failing.

I have also adjusted the handbrake. This shows no difference in the regular brake.

I have cleaned al the internal parts while replacing them.

I'm out of options and hope someone has got tips for me how to proceed!

 

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New cilinder with the old pads.
The oil at the bottom is from bleeding the cilinder