64 C20 Dual Master Cylinder Conversion – Drum/Drum Questions

[theastronaut]

Here's the math on what changes with a booster, or with different diameter master cylinders. When you add a booster you have to change the pedal ratio (you lose leverage) and the bore size goes up (you lose hydraulic pressure), so if the booster dies you're way down on manual braking power. The total line pressure of a typical booster setup and a correctly sized manual setup isn't all that different, its just that the booster adds more assist at lower pedal input so it's a little easier to stop in normal day to day traffic, but they're about the same in a panic stop. I prefer the linear input to output of manual brakes, and braking power doesn't vary with engine vacuum. Remember also that drum brakes are self energizing due to the shoe/pivot geometry so they need less input pressure to be as effective as discs. Drums work very well when they're done correctly.


Another thing I never see mentioned about older non-abs systems when upgrading them; you do not want an easy brake pedal if you do not have abs. If the pedal effort is low/easy then it'll be very easy to lock up the brakes and slide uncontrollably in a panic stop situation. Newer cars with abs can get away with overly boosted brakes to make it easy for a frail grandma to stop the car, but abs kicks in to keep it from locking up. It should take some pedal effort to lock up the brakes, enough that you have to work at it a little so you can modulate the pedal right at the point of maximum tire traction. "You" are the abs, and a linear pedal response with manual brakes makes braking as hard as possible without locking up easier and more intuitive.

Manual pedal ratio is generally around 6:1, power brakes are generally around 4:1. My stock '66 C10 has a 6.25 ratio. The higher the number, the more leverage the pedal has on the master cylinder.

When you convert to power brakes you do two things to take away manual braking power- reduce the pedal ratio by about 35%, and you generally go from a 1" bore MC to a 1.125" bore MC, reducing output PSI by 20%. The booster is supposed to more than make up for that, right?

With a stock 6.25 pedal ratio and 100 pounds of input force on the pedal you get 625 pounds of force into the MC, which is hydraulically multiplied via the MC's piston surface area of .785 square inch. 625 pounds in turns into 790 PSI at the caliper if you have the typical 1" bore MC.

With a power brake conversion, 100 pounds of force on the pedal gets multiplied by 4 via the pedal ratio for 400 pounds into the pushrod/booster. CPP says a 7" dual diaphragm booster adds 493 pounds of force for 893 pounds of force into the MC. The typical 1.125" MC will turn 893 pounds of input force into the same 893- the bore size has 1 square inch of area so it doesn't multiply the input force any so you have 893 psi at the caliper. This is about 13% more psi than the typical manual setup ** BUT ** this is assuming 23" of manifold vacuum. How many trucks actually make 23" of vacuum? That 13 % advantage quickly fades in the real world.

What happens if we stay with manual brakes but drop down to a smaller master cylinder bore to multiply input force more? With a 15/16" bore you get 905 psi at the caliper, and 7/8" would have 1041 psi. Brake pedal input to output is totally linear and not variable with manifold vacuum, and they work 100% the same with the engine on or off. It's cheaper and easier because pedal modifications aren't needed.

But, everyone says you need power brakes "because they stop better" and brake companies need you to keep believing that so you keep buying their aftermarket "upgrade" parts...

The one benefit to power brakes is that they can have less pedal travel than manual brakes, because of the shorter pedal throw due to the lower ratio, and because the bigger MC bore displaces more fluid for the same amount of travel. Is this an issue in the real world? Its not on my '66 with the stock 6.25 pedal ratio, 1" bore MC, the usual squarebody D52 12" discs in front, and stock rear drums. The pedal drops a normal amount and pedal effort is pretty normal too. If you have air in the lines then you'll have more pedal travel since air is compressible, but you shouldn't have air in the lines if you bleed them correctly. If you have bigger/multi-piston calipers then you'd need to design the whole system around the amount of volume/pressure they need. But for the typical D52 disc setup, a manual 1" bore MC works great and makes plenty of hydraulic pressure without the pedal travel being any more than normal.