there are no selective pressures on pseudogenes.
Once the vit C synthesis route was broken, and the chimp-human LCA had the GULOP fixed in it's population, there was no selective pressure. There basically is no garbage collection routines in genetic mechanisms, if a gene doesn't produce something useful then the selective pressure on that gene is effectively zero (genetic drift etc still operate on it however)
The interesting thing about the GULOP, it that since the gene is fixed we know that that LCA had a VitC rich diet and that the broken enzyme didn't burden those individuals who carried it. Otherwise it could never have driven to fixation.
there is lots of information on this pseudogene, perhaps the most interesting i found quickly was:
http://www.d113.lake.k12.il.us/hphs...weiland_web_page/evolworkshop/pseudogenes.htm
a handout for a student lab on the topic.