This is a sarcastic paragraph. He is comparing the bleeding with leechs, which was a very common remedy at that time, with a cut his father made on the sack. "Sanguijuela", leech, means also a person who, little by little, gets whatever he needs. So, he is comparing his father with a leech.
El Lazarillo de Tormes is a very interesting book, but it may be a bit difficult for beginners, since it was written about 16th century.
Sorry, I forgot to explain what a "sangría" is. Yes, this is an alcoholic beverage with wine, fruits, sugar, brandy, orange, some lemon and cinamon (people change the recipe as they like).
Sangría: