Shaving brushes made of natural hair allow for a pleasant shaving experience - day by day. And the are made to last a lifetime; provided they are used correctly.
Soap is strongly alkaline. Once even smallest amounts of soap remain in the brush hair, they cause extensive damage to the hairs which will break at some point. Take your hand and stroke the dry hair: If the dry brush hair raises dust or has a whitish or greyish film, it certainly contains soap residue.
Therefore: It is strongly recommended that you rinse your brush after usage with clear water for a couple of seconds. After that, gently move your fingers through the brush hair whilst continued rinsing in order to clean it properly, so that especially on the inside of the brush no soap is remaining.