volare brake caliper failure

[wvcab]

put a volare front end under the panel, and bought a set of calipers off ebay for it. put maybe 1500 miles on it, and the drivers side caliper started leaking around the piston. so i replaced the caliper last month, and today, it started leaking again in the same spot.  The replacement caliper came from NAPA, so it has a warranty, but i wanna know what else could cause the failure.
when i installed to clip, i put new rotors (from NAPA) on it, replaced all the bushings, wheel bearings, brakes.  The pads look new, the rotors dont have any groves, and the van stops in a straight line.

ideas :?:

[Mikej]

Is it a rebuilt one?  Do they have a new one or a better quality part. Seems like you have to ask any more or they give you the competitive one (cheap).