Baba Metzia 40b - Old Keilim Don't Absorb

The Taz (y.d. 93:2) cites the Beis Yosef quoting Sha'arei Dura that if one cooks meat in a dairy pot that had milk sitting in it all night, so long as the pot has been used before so that the walls have fully absorbed to capacity, the meat would be permitted to eat. The rationale is that the walls of the pot, since they have absorbed to their capacity couldn't possibly absorb any of the milk that was in it overnight, therefore the pot is considered not to be a ben yomo (assuming that 24hrs had passed from the last time milk was cooked in it) and doesn't assur the meat. The Mordechai proves this concept from our gemara which says that an old pot - כיון דטעון טעון. Meaning, since the walls of the container have already absorbed oil, one can assume that the container didn't continue to absorb the oil. Therefore, the nifkad must return the full quantity of oil that he received and can't assume that the container absorbed some of it.

The Bach extends this din to a case where the milk sat in the pot for more than 24hrs where we generally have a concept of kavush k'mivushal - nevertheless, the pot doesn't have the capacity to absorb any of the fresh milk since the walls were fully absorbed with the old eino ben yomo milk that was previously cooked in it. The Taz strongly rejects the extension of the Bach. When milk sat in the pot for more than 24hrs we have a concept of kavush k'mivushal which means that the walls of the pot will "spit out" some of the old milk and absorb in its place some of the new milk. Therefore, the pot becomes a ben yomo of dairy and will thereby assur the meat. It is only in the case of the Sha'arei Dura where the milk didn't sit in the pot for a full 24 hrs so that the walls of the pot didn't "spit out" (poleit) any issur - that is why we can assume that so long as they were filled to capacity, they couldn't have absorbed any of the fresh milk. 

The Chavos da'as (93:3) and Pischei Teshuva both point out that even without the distinction of the Taz, our gemara cannot serve as a proof to the case of the bach where the milk remained in the pot for more than 24hrs. When milk sits in the pot for more than 24hrs we have a concept of kavush k'mivushal, therefore even if the pot cannot absorb any more actual milk, it can absorb the "ta'am" (taste) of the fresh milk rendering it a ben yomo pot. It is only in the case of the mafkid and nifkad where the fact that it has already absorbed to capacity will allow you to assume that none of the oil got absorbed, because in monetary issues we are only concerned with the mamashus of issur, not the ta'am of the issur.