There are several things that can cause this pulling problem. From worn suspension components to glazed pads to uneven pressure to the calipers. The rear wheels have little effect on the braking operation since when the brakes are applied, the weight of the vehicle will shift to the front and the front brakes will provide about 80 to 85% of the braking operation. The seals that you are referring to should have a small amount of synthetic grease or brake grease (you can pick up a small packet from AutoZone for about $2) on them to allow the caliper to return after pressure is released. Use only a small amount of this grease on the seals. Also, look at the condition of the metal brake lines. The chemicals that make up brake fluid is known to be hygroscopic. This means that it readily takes up and retains moisture. This moisture in the brake system causes many things and one of them is rust. The wear and tear of the rubber components, small metal particles, rust particles can cause the metal brake lines to act as restrictors and cause the differential pressure between the left and right sides causing it to pull. Air will also trap in high spots in the brake system while fluid will go to the low spots. When pumping the brakes to bleed them, the air will move back and forth and when you release, it will migrate quickly back to the high spot. With spongy brakes, air is most likely the culprit, fluid blowing by seals is the other. You may have one side that has a small pocket of air in it that is not noticeable. This can also cause direct pressure to one side and the side with the small air pocket in it will have lower pressure to it to extend the caliper. The difference between air and fluid is that you can compress air (which is why it seems spongy) but you cant compress liquid (fluid). I discovered this process many years ago with a 1st gen Bronco. I blew a steel line that rusted from the inside out in the middle of no where. While I was rebuilding my 79, I pulled all the steel brake lines and cut a few open and found some serious rust forming in them. One way I found to get all the air and crude out of the system was to use a Mighty Vac vacuum pump. I had a friend keep the master cylinder full, pinned the pro valve and start pumping. This kept a steady stream of fluid coming down the line. I saw a lot of air and crude come through even after I had the system bled (I was still having a spongy brake). Brakes are firm and solid now and they do not pull. Also, on the rear brakes, adjust the shoes all the way to the drum prior to bleeding the brakes. If you can't adjust them, take the drums off and with a small block of wood on each side to protect the pads, use a speed clamp to keep them from flexing during the bleeding. This takes out any flexing in the system. Readjust them after you are done bleeding the system. Start bleeding them from the fartherest cylinder (ie, left rear, right rear, right front, left front). Anyway, that's my .05, sorry this is long but good luck.
