MANUAL BRAKES TECH NOTE: Generally a 1" bore master cylinder is best with manual brakes. Manual brake master cylinders have a smaller piston bore diameter to maintain sufficient fluid pressure in the braking system. They also have a deeper hole for the brake pedal pushrod. TECH NOTE: You can use a manual brake cylinder on a power brake system, but not vice-versa. The power brake cylinder's larger piston bore cannot supply the required level of hydraulic pressure required by manual brakes. POWER DISC/DISC TECH NOTE: Generally it is best to use the the 1 1/8" bore when using power assisted disc/disc brakes with larger aftermarket brake calipers. For OEM disc/disc brake applications, the 1 1/8" bore is usually the way to go. POWER DISC/DRUM TECH NOTE: Generally it is best to use the 1" bore for power disc/drum applications. TECH NOTE: Use a deep hole model for manual brakes and a shallow hole model for power brakes. TECH NOTE: Dual diaphragm boosters will provide easier pedal pressure over single diaphragm models. TECH NOTE: The above information is a general guide. With custom brake applications many variations can be used. Call us if you have a question as to which is best for your application. CLICK ON THE PICTURES FOR DETAILED INFORMATION Tuff Stuff 2121NA-1 7" Dia-3/8" Ports Single Diaphragm Disc/Drum or Drum-Drum Tuff Stuff 2122NA 7" Dia-9/16" & 1/2" Ports Dual Diaghragm Disc F Disc R Tuff Stuff 2122NA-1 7" Dia-3/8" Ports Dual Diaphragm Disc F Drum R Tuff Stuff 2122NA-2 7" Dia-91/16" & 1/2" Ports Dual Diaphragm Disc F Drum R Tuff Stuff 2123NA 8" Dia-9/16" & 1/2" Ports Dual Diaphragm Disc F Disc R Tuff Stuff 2123NA-2 8" Dia-9/16" & 1/2" Ports Dual Diaphragm Disc F Drum R Tuff Stuff 4750 Universal Kit Brake Booster Rod Extension 1973-1993 GM Truck Chrome Plated Brake Booster & Master Cylinder Combo HOW TO ORDER: 1. If there is an applications box, click, then scroll the applications box to find your vehicle. 2. Click on your vehicle. 3. Click "add to cart" 4. Enter your information and submit order. 5. If there is no applications box, then just click "add to cart", then enter your information. HOURS: 11:00 am to 6:00 pm Monday - Friday Eastern Standard Time We welcome CANADIAN customers! For ordering information, click on "Orders To Canada" at the top left side of this page. We will not ship to freight forwarding - expediting companies. PAYMENT METHODS: PayPal, Visa, Mastercard, Discover, American Express, money orders.