Brushes used with tar are difficult to clean completely. Wipe out excess first, then use the cleaner recommended for the product, or reserve the brush permanently for tar work.
Tar is hard on brushes because it is sticky, dark, oily, and slow to leave the bristles. The easiest method is prevention: use an inexpensive brush or a dedicated tar brush and do not expect it to return to clean paint work afterwards. A brush that has touched tar will often keep colour and smell even after cleaning.
Before any solvent is used, scrape and wipe as much tar as possible from the brush onto scrap wood, cardboard, or a disposable surface. This reduces the amount of cleaner needed and stops the brush from becoming a thick black lump. Then use the cleaner named by the tar product rather than mixing solvents at random.
Cleaning waste, oily cloths, and solvent containers need careful handling. Do not pour tar residue into drains or leave solvent-soaked rags in a pile. If the brush will be used again soon, wrapping or suspending it according to the product instructions may be more practical than trying to make it spotless.