How can I fuse separate, small pieces of ABS plastic that can withstand pressure?
PlasticWeld is a hand-mixable, fast-setting epoxy putty that forms a durable bond to most major plastic types. *After mixing, it forms a polymer compound that can be molded or used to build up and repair just about anything made from plastic. When cured, it can be sawed, drilled, carved, sanded and painted. PlasticWeld has a 20-25 minute work life; functional cure occurs in 2-3 hours. PlasticWeld cures to an off-white color, is rated at a tensile strength of 350 PSI. Does not adhere to polyethylene, polypropylene or some other plastics. Test for adhesion first.

Great for: 
Rigid &amp; Semi-flexible Plastic
ABS, PVC &amp; CPVC Pipes
Automotive Trim &amp; Bumpers
Fiberglass Parts
Vinyl Siding

Found at most automotive and hardware stores:Products link to O'Reilly's

DETAILED DESCRIPTION
Repair and rebuild any rigid and semi-flexible plastic including ABS, PVC and CPVC
Bonds instantly
Superior strength
Will not shrink or pull away
Rebuild automotive trim, fiberglass parts and vinyl siding
ADDITIONAL DETAILS
Set Time (hr): 0.25 Hour
Cure Time (hr): 3 Hour
Volume (ml): 60ml
Volume (Oz): 2 Ounce
Cure Color: Off White
Material: Epoxy Resin
Color: Off White
Solvent Resistant: Yes
Temperature Range (Deg F): -67 To 300 Degree
Strength Rating (psi): 350 psi
Strength Rating At 400 Degrees (F): 350 psi
Water Resistant: Yes
Waterproof: Yes
Weatherproof: Yes
Type: Epoxy Stick



EDIT: Adding instructions here so it is not lost in comments.

INSTRUCTIONS
CUT:
Remove required amount of putty.
MIX:
Thoroughly knead putty with fingers to a uniform color.
APPLY:
Press putty firmly to the surface to be repaired.