For sale: a used CamelBak Octane 18X hydration backpack in like new condition with a 3 liter reservoir and hose. Black with yellow accents, weighs 24 ounces on my scale. It has a zippered center section to expand the capacity from 13.5 liters to 18 liters. $38 including US shipping and PayPal fees.
Vendor's description:
The CamelBak Octane 18x 100 oz Hydration Pack is an expandable hydration pack for fast moving trips on the mountain bike or on the run. The ultralight materials haul your gear without adding the weight. The three liter Antidote reservoir carries all the water you'll need for three or more hours on the trail. The main compartment can be packed with up to 18 liters of extra layers, food, or tools. If you're running light, zip up the front and it compresses down to 13.5 liters of space, streamlining everything on your back in a neat little package. A wide, winged hip belt puts the load low on your back. The Air Mesh back panel and shoulder straps keep you cool while pushing it hard hour after hour.

FEATURES of the CamelBak Octane 18x 100 oz Hydration Pack

Reservoir Hanger
Air Mesh back panel
Independent Suspension Harness
Integrated harness Pocket
25mm / 1" with Cargo Pockets
Ultra-Light Materials
Media Pocket
Expandable Cargo Space
Bungee attachment system
Front and Back reflectivity
Center expansion zip
Essentials pocket
Quick stash overflow storage
Harness: Ultra-light 3D Mesh Independent Suspension with gel pocket and Slider Sternum Strap
Designed to Carry: Extra layers, ultra-light weather protection, food, head lamp, trail maps, compass, trekking poles

SPECIFICATIONS of the CamelBak Octane 18x 100 oz Hydration Pack

Hydration Capacity: 100 oz / 3 liter
Frame Size: 16" / 41 cm
Total Capacity: Open: 1198 cubic inches / 18 liter + 3 liter Reservoir, Closed: 823 cubic inches / 13.5 liter + 3 liter Reservoir
Pack Only Weight: 17.9 oz / 507 g
Dimension: 18 x 10.5 x 8.5" / 46 x 26 x 22 cm
Fabric: 70D Diamond Box Rip, 230D Taffeta and 420D Nylon with DWR + 1000 mm PU