@ama1 Agreed.
I can do 5 x 40kg at 80kg. The trick to weighted pulls (and I swear by this) is to reverse pyramid them. Do 3 reps at max weight then drop 10% weight but add 1 rep. 5 sets.
E.g
3 x 20
4 x 18
5 x 16
Nothing in this world will fuck your back up like reverse pyramid weighted...