Recent models for evolved Low Mass Stars (with M 3 M ), undergoing the AGB phase assume that magnetic flux-tube buoyancy drives the formation of 13 C reservoirs in He-rich layers. We illustrate their crucial properties, showing how the low abundance of 13 C generated below the convective envelope hampers the formation of primary 14 N and the ensuing synthesis of intermediate-mass nuclei, like 19 F and 22 Ne. In the mentioned models, their production is therefore of a purely secondary nature. Shortage of primary 22 Ne has also important effects in reducing the neutron density. Another property concerns AGB winds, which are likely to preserve C-rich subcomponents, isolated by magnetic tension, even when the envelope composition is O-rich. Conditions for the formation of C-rich compounds are therefore found in stages earlier than previously envisaged. These issues, together with the uncertainties related to several nuclear physics quantities, are discussed in the light of the isotopic admixtures of s-process elements in presolar SiC grains of stellar origin, which provide important and precise constraints to the otherwise uncertain parameters. By comparing nucleosynthesis results with measured SiC data, it is argued that such a detailed series of constraints indicates the need for