Save old, dried latex paint brushes using common acetone solvent to remove dried latex paint from brushes. But too much paint for too long could harm the bristles.
Use a non-plastic container as acetone will dissolve some plastics. Also consider rubber gloves because it will irritate your skin.
Pour enough acetone into a glass or metal container to cover the bristles of the paint brush to soak for 10-15 minutes.
Massage the bristles with your hand to aid loosening the paint – a comb may help.
Rinse the brush with clean water.
Lay the brush flat to dry, not on the bristles, to avoid bending them.
Or try:
Use warm soapy water which is less harsh than acetone, but not as effective for removing dried paint.
Paint thinner can be used though it also is strong and can damage the bristles.
Commercial brush cleaners are specifically designed for latex paint.
Cleaning brushes immediately after use makes them easier to clean.
Use a commercial brush conditioner or a light coating of mineral oil to keep the bristles soft and pliable.