yup.
ebay - get a Berghaus Cyclops Roc or Cyclops Crusader - the Roc is about 85ltrs and weighs about 1.2kg, the Crusader is about 90ltrs, takes two 15ltr zip-on side pockets and weighs about 1.8kg.
you'll get both in very good condition for less than £60 or so. they will be 15-30 years old and will last another 30 years. the back system is simple, its old fashioned, but its bizaarely comfortable.
if you fancy something that looks a bit less agricultural you could go for the Berghaus Cyclops Vulcan or the Karrimor Condor - again, Ebay - they have a more padded, complex back system and therefore weigh a bit more. the Vulcan weighs about 3kg and the Condor will be something similar - the Condor is huge, something like 120ltrs, but its let down by its lid, which is about the size of a barn door, meaning that when the rucksack is 30% empty the lid rattles around like a demented thing.
my favourite is the Cyclops Roc, its the lightest of the big rucksacks, and it will take a 1 man tent, sleeping bag, air bed, and 6 days food, fuel and clothes for a winter trip across Scotland with ease, or tent, sleeping bag, air bed and food, fuel, and clothing for 10+ days in summer. if i need more stuff i just bungee some drybags to the sides - you can get side pockets for them, but they're only 5ltrs and not worth the effort. i also have an Alpkit Gourdon 20ltr waterproof bag that i stuff under the lid, i use that as a daysack if i want to dump the big rucksack somewhere and go and have an explore...
personally i'd ask yourself if you really need a 100ltr+ bag - if you fill it can you really carry it any distance? can you cross a river with it, can you climb over a stile or a fallen tree with it?
