I work in a museum and deal with this type of conservation issue daily. The yellowing is caused by the inherent acidity if the paper. Top coats are a bandaid approach that do not address the real problem, and the deterioration will continue. There is a deacidification liquid on the market called Bookkeeper, it is available through many archival supply houses. One of the better companies is gaylord.com (legitimate site I swear) Unfortunately it is not cheap. It neutralizes the acid with magnesium oxide and leaves what is called an "alkaline buffer" to act as a reserve to neutralize any new acid formation. It needs to soak into the paper, so if you have done any varnishing it needs to be used on the back side of the paper.
The brittleness can either be from the acid deterioriation, or could be from UV light. UV light wavves are very tight and can actually sever molecular bonds causing the object to literally fall to pieces. If it is UV damage you can halt the process by storing items in total darkness for at least a month (6 months is the museum standard) to halt the molecular excitement caused by the light.
Unfortunately, either way, the damage is irreversible. All you can do is arrest the development of further damage from these sources. As is posted above, the best approach is prevention, acid free paper, glue, paints and UV protectant top coat.
Hope this helps out, sorry it is such a long post.
chris