It shouldnt need to be bled ever, so it probably needs a new seal. After a few million shifts the edges are rounded off and it will start to suck air on release or leak down.
I had a truck, the slave was aluminum with a slightly pitted bore. replacement was $$$ from the stealer. But a few cents for a new seal with fresh new edges every couple years kept it going for a looong time. (This slave happened to take a regular brake cup so that was cheap)