This Replacement Rubber Kit will rejuvenate and keep your hydrometer set in good working condition. Use this red natural rubber kit to replace cracked and dry-rotted rubber on your battery, antifreeze, windshield washer fluid sets. Use our black nitrile rubber set for your API, diesel, home heating fuel, and Nitromethane testers. This item is not intended for use with other brand testers. For best results, use only Brady Instruments brand testers.