Just guessing, but l suspect there is difference in
Pr between the fwd and aft drains. The fwd are ok with delta P to work whilst the aft need a bit more assistance with the aft facing mast.
Landed in Riyadh once and the fwd GMC was all dark. So asked engineer to check it out.
Turns out it was covered in prawn juice. We had 11 tonnes of prawns that were supposed to be frozen in the fwd hold. They were not. They had been left on the tarmac in 45 degrees for many hours prior to loading.
The juice came out through the drains and on to the camera lens, obliterating the view.
The aircraft was pretty high when we got back to base. Total mess!