Nah, you don't need to mess with a vacuum pump. Results would not be definitive anyway since air might sneak past a restriction in the line a lot quicker than viscous brake fluid would, thus yielding a false negative. The simple way to test this would be to have a helper apply the brakes and then release them. Shortly after the brakes are released, you crack open the bleed valve on the caliper in question. If fluid sprays out under pressure, you know you have a fluid return problem - either in a rubber line or at the master cylinder. More than likely the line. The only times I've seen a MC cause this problem was after a rebuild using out of spec cups that covered the return hole or else using a drum brake MC for disks - and like Tom pointed out - you would have more than one caliper involved if the MC were causing the problem.

If fluid just dribbles out via force of gravity then you know the problem is with the caliper itself. Hopefully the latter is the case since calipers and pads are new. Caliper surely would be replaced under warranty and you could probably shame the supplier into replacing pads & rotor too since the defective caliper is what trashed them.

If it does turn out to be the flexible line, it's not a huge deal money-wise and like Tom said - a good chance to upgrade to SS lines which will give you better feel when braking. Make sure you get a five line kit with real stainless steel not Kevlar - it's strong but not nearly as abrasion resistant as SS - and you definitely don't want the ones wrapped in SS-colored plastic - they're just for looks. Goodridge is a brand you can trust. There are probably others but I don't have any experience with them. Never used anything but Goodridge.

And no, it's not your ABS causing this. BTW, did you know you can get your ABS unit repaired for less than $100?