Brake positioning switch may be sticking
I'm speculating
Judder with raspberry blow back..look under the master cylinder for seepage air leaks will look dusty but sort of fresh and obvious.
Pistons may be sticking or not correctly repositioned when pushed back in.
Check and clean cylinders, assuming the master brake cylinder servo was flushed a mechanical operation prior to new dot 4 fluid being put back, this process cleans lines of water and rust particularization.
Each wheel brake has to be separately bleed and primed it's a two man job one does the pumping in the van with brake pedal, the other is looking for a clean flow of fluid without airlocks in the visible plastic primers line.
I'm working off my lap top so no photos sorry.
Check that the rear brakes are not grabbing it may be your hand brake ABL lever is a tad over adjusted and the park brake is catching on the rear parking pads or they are seriously blocked with contamination such as brake dust and road debris,which results in uneven braking at low speed.
Look at the top of your differential housing there is a line that goes to the chassis rail that has the central mechanical housing for your rear brake system the line from the diff conects under the chassis rail to this unit its a pressure brake porting valve, then look at the same line at the top of your diff it should be damp not leaking???if so you are losing brake pressure at this section resulting in air in the lines,its rare and almost unheard of.
Left side of van rear forward from ABL level check adjustment springs are working you can tell when looking as driver pushes the brake pedal.
Also you have four lines coming into this section make sure they are tight and not leaking.
If you can not resolve what is occurring this needs to be re calibrated on a scanner to find either electrical or sensor fault. ECU fault codes are not stored on all model Sprinters ECU I think yours is or should be but it depends on the scanners ability to pick up,dependent on type and data configuration.
Hope this is of some use to you ,as it may not be the cause of your brothers problem.
Richard