One of the greatest inventions for vehicle safety since the advent of hydraulic brakes has been the tandem or dual-circuit master cylinder. Usually made from cast iron or cast aluminum, the modern master cylinder receives its input pressure from the power booster, and as the primary piston inside the bore travels it creates pressure between it and the secondary piston. Both pistons displace a fixed amount of brake fluid creating line pressure at the calipers or wheel cylinders. (Refer to the cutaway photo of a typical master cylinder) If the primary piston should fail, pressure will still be generated using the secondary piston and the vehicle is able to slow down and stop using two brakes rather than none. The single-circuit master cylinder was in use until the mid-'60s; in 1967 it became mandatory for U.S. automakers to employ the dual circuit system.
Not all master cylinders are created equal. There are master cylinders with smaller bores for those who prefer manual brakes and master cylinders with slightly larger bores for the majority of vehicle owners who run power-boosted brakes. Among power-boosted systems, master cylinders will vary as well. For instance, a master cylinder powering a disc/drum set up is different than a master cylinder powering a disc/disc set up. That difference is usually bore size. Disc/disc set ups require more psi, which usually requires more reservoir volume for the brake fluid. Whatever the user's choice, making sure the booster and master cylinder will fully power your vehicle's brakes is of utmost importance and having mismatched components can make even the best parts perform poorly.
Hydroboost! When I was kid I used to think it was a button that made the Knight Rider Trans Am jump buildings. When it comes to brakes, hydroboost is really nothing new. It was first used in domestic production cars in the late-'70s, using the power steering pump to increase vacuum pressure to the brake system's master cylinder. Now hydroboost represents the "latest" in brake technology. But why would you need it in a classic car?
Two reasons: The first that comes to mind is cam size issues. We all love cams that have more lumps than grandma's oatmeal, but a vacuum-powered brake booster may not like it at all. Cams with a lot of overlap can severely affect the engine's vacuum output at lower rpm. This is a bad thing if you've got power brakes and like to stop.
Another issue that makes using hydraulic boosters attractive is hood, fender and valve clearance-or lack thereof. Just ask any Tri-Five owner. Despite their deceptively large engine bay capacity, there is precious little room for the placement of larger aftermarket boosters under the hoods of these (and many other) cars.
While they do take up less space, they can weigh more than a typical vacuum booster set up. Most hydroboosters receive their power from the power steering pump, though now there are those that work off an electric pump. There are two different hydraulic boosters that we wanted to take a look at.