1995 Mercury Sable brake reocurring problem

I have a 1995 Mercury Sable I bought used Aug '04. The brakes on the left rear have been a constant problem since I purchased the car. The reller replaced the brakes just prior to my purchase. He has paid to have them worked on 3 more times since.
The complete and subsequent brake repairs have not lasted more than 2 months before the pads have been destroyed only on this one wheel.
I would be grateful to hear any opinions about what is possibly being overlooked.
P.S. The brakes have been spongy from the very beginning.

Sticking caliper.
Bad flex line(s).

Thank you for your reply KIM.

We've replaced the calipers 3 times.
the brake line did break and had to be replaced, but the same problem has returned twice since the line replacement.

KimMG...
Sorry I sent my reply really before I was through thinking - DUH!
Bad Flex Lines. I check into that.
Thnx.
C

The flex lines can deteriorate on the inside and cause blockages, acting like a one-way valve.
Check the parking brake to make sure it is functioning properly. If it is not releasing it could be causing problems. Are you having trouble bleeding the rear brakes (spongy pedal)?
Last note, although unlikely, new parts are not always good parts.

Thanks KIM. That's probably been what has happened. The brake line that failed was on the right rear. I don't know if the left has been replaced yet. I put in a call to my mechanic and am awaiting his reply.
Yep, about the new parts not being always good.
Thanks again!
C.

The problem was in the rubber link of the brake line. We dissected the part we removed. The blockage was right in the area where the line is held in place with a bracket. We could not even run a paperclip through the blockage!
Thank you for your encouragement and help. My mechanic thought it was a chuckle that I was able to find out the problem and tell him how to fix it!
He said it had happened to him once before on a Ford F150 pickup.
thanks again!
Cecilie