When the brake pedal (or clutch pedal) is too hard to push, driving your car becomes difficult, and downright dangerous. Disc brake conversions are great, but even converting to a power-assist braking system can alter the feel of the brake pedal, and not all vehicles are capable of using power brakes, so ensuring that the pedal ratio is correct is paramount to practical and safe operation of the vehicle.
One of the projects in the Red Dirt Rodz stable is a 1963 Buick Le Sabre wagon with twin-turbos. Originally, the wagon had power brakes with a single-reservoir master cylinder feeding four-wheel drums. The front drums were upgraded to discs, but the turbo system design meant that the power booster had to go. Which we replaced with a new factory replacement double-reservoir manual master cylinder. The result of this swap was not acceptable; stopping the car became downright scary.
A common issue with master cylinders is that there are quite a few options, especially in the bore size. When it comes to master cylinders, the larger the cylinder bore, the harder it is to press. This is due to the fact that there is more fluid in front of it. The rule of thumb for manual brakes is no larger than 1 inch, and in reality, 7/8 inch is about optimum for factory-type master cylinders. Power-assist master cylinders have a lot of help to move that piston, so they can be larger.
We swapped at least 5 MC units, and none made even a hint of difference. During the initial installation, we pulled out the pedal and drilled a new hole for the pushrod. What we did not do, was actually measure and calculate where that hole was put, we just positioned it as high in the car as the master cylinder would allow. After none of the recommended solutions helped, we had to get serious.
Pedal ratio is the difference in length between the pivot (fulcrum) of the pedal to the pushrod hole (Y) and the fulcrum to the center of the brake pedal (X). A power system should have a ratio between 4 and 5:1, where a manual system should be between 5 and 7:1. Consider this: a master cylinder with a 1 inch bore, and a pedal ratio of 6:1 with 100 pounds of pedal pressure, yields 600 pounds of pressure at the master cylinder. Cut that pedal ratio to 4:1, and the pressure at the master drops to just 400 pounds, that is a significant difference, and in a manual brake car, will increase how hard you have to stand on the brakes.
We got under the dash of the Buick and took some measurements, and the results were astounding. The Y measurement (fulcrum to pushrod) is 5.5 inches; while X (fulcrum to center of the pedal) is 14 inches. That is just a 2.54:1 pedal ratio, which is ridiculously low for a power system, much less a manual master cylinder. The biggest problem that we face on this car is the position of the pedal fulcrum and the floor of the car. We can’t completely re-engineer the car, but the master cylinder placement is too low on the firewall. The pedal itself is just an inch off the floor of the car at its lowest point, so we can’t just make it longer. Major surgery is required, but we need to make sure it will work before we cut.
The master cylinder needs to move up on the firewall, but it can only go up about 3 inches before it’ll hit interior structures that can’t be moved, so we drilled new holes through the firewall and support brackets. The support structure in the car did not allow for thru-bolts where we needed them, so a support plate was fabricated using 10-gauge steel, with two studs welded in place to bolt down the MC. We sealed up the original MC hole with a piece of sound matting from Eastwood.
The pedal was removed from the car for modification. Our target ratio is 7:1, however the limits of the pedal length and MC placement would not allow for it. To maximize the pedal length, the pad was cut off, moved down 1 inch, to the top of the pedal, and welded in place, along with a gusset for strength. This increased the overall X length to 15. The pushrod hole was placed 2.5 inches from the center of the fulcrum, which yields a pedal ration of 6:1.
The new placement of the master cylinder required making new hard lines for the brakes. With that completed and the brakes bled, we test drove the car. Unlike the previous impossibly hard brake pedal, the wagon now has a much softer pedal that actually stops the car without the need to use both feet and nearly break the seat. If your car has a hard brake pedal and you just can not seem to figure out the solution, take a few measurements. The pedal ratio is likely the culprit.