I agree as well most likley BF. If you have a motive bleeder install it on the rear section on the M/C and pump it up to 10-12 psi. Watch the gauge if it creeps down you have a leak in the rear system, and most likely at the caliper pistons.
The original and many of the aftermarket SS lined calipers use lip seals which really are fine as long as you maintain the system. That means bleeding the system after storing the car for 3+ months, checking the bearing endplay and if over 003, maybe 0035" rebuilding them, checking rotor runout and getting it under 003". That's all you need to have a solid, sealed brake system, Fancy rotors,calipers,pads,lines are not needed for street driven cars.
If your brake system hasn't been worked on in a few years and you have a caliper leaking then I would plan on going over the system, replacing all the rubber hoses and checking like I said.
The rear trailing arms have seals at both ends of the spindle. If they have been rebuilt it's possible that too much grease was uses, cheaper seals used, or poor assembly procedures used. The grease could then weep out, usually shows up in the parking brake area under the rotor. If you still have the rivets in the rotors chances are very high they've never been touched, although there are a few places that will re-rivet them but that is not necessary.
Good luck.
