Different types of brake fluid should never be mixed. Glycol-based brake fluids, such as DOT 3, DOT 4, and DOT 5.1, are chemically compatible, but mixing them can change the boiling point and brake performance. Always check your vehicle's owner's manual for the correct brake fluid type. You must NEVER mix silicone-based brake fluid with glycol-based brake fluid because they do not blend and can form gel-like sludge that can cause brake component failure.