Brake Size Sense - Kopycinski's BrainPosted in How To: Suspension Brakes on August 22, 2014
You swapped out your axles for bigger, badder units and you increased the size of the brake components on the axle as well. Well, you’ll typically find that now you’ve got insufficient fluid flow from your stock master cylinder to fully engage the new brake calipers or wheel cylinders without an excessively long pedal stroke or the need to double-pump the pedal. Simply put, the slave cylinders need more fluid than the master can provide in a safe, single pump.
The stock master cylinder is sized to offer comfortable pedal travel with ample margin, keeping the pedal from bottoming on the floorboard. When you increase the slave sizes, it’s often time to increase the bore size of the master cylinder to push more fluid to the calipers and/or wheel cylinders. However, if you choose too large a master cylinder bore, you’ll end up with a very short pump stroke. More importantly, you lose mechanical advantage as you increase the master bore size, and the result can be brakes that require excessive leg force to stop adequately. It’s best to go larger only if you lack enough fluid movement (i.e. too long of pedal travel).
The fluid flow and mechanical advantage offered are directly dependent on the bore surface area of the master cylinder piston. For example, if a 13/16-inch master cylinder is replaced with a 1-inch bore master cylinder to match up with larger calipers, the piston surface area increases by about 51 percent. The larger master cylinder will move about 51 percent more fluid with the same pedal travel, but will require a similar increase in force at the pedal (leg strength) to obtain the same braking pressure. There’s your tradeoff.
When it comes time to swap master cylinders or calipers, a little math can go a long way toward making sure you get a brake system that performs to your expectations. As the bore of the master cylinder grows, fluid movement increases but pedal effort rises in proportion. You can, more or less, restore your pedal movement behavior with a master cylinder swap. If your caliper bore surface area grows by 20 percent, you’ll want to look at increasing master cylinder bore surface area by about 20 percent.
Just like the surface area calculation used for the master cylinder bore, a similar calculation is valid on the caliper side but with a larger piston providing greater mechanical advantage. For a four-piston fixed caliper, we would use the surface area of two pistons, not four. The pistons on both sides of a fixed caliper need to move only about half the distance the one piston on a floating caliper does.
Typically, but not always, the master cylinder on a front disc/rear drum vehicle will have a larger front reservoir for the disc calipers as opposed to the smaller reservoir for rear brake cylinders. Disc brakes typically require more fluid to actuate calipers than drum brakes require for the wheel cylinders.
To optimize your brake setup, it’s best to use a master cylinder size that provides reasonable pedal travel and maximum mechanical advantage. We’re trading a large pedal movement with light applied force for a very small brake pad movement with large applied force. In other words, we’re also using a relatively small bore master cylinder with long piston stroke to feed larger bore calipers to make a much shorter piston movement.