Brake Bleeding Anomaly on my 66

[Grounded63]

You dont add a vacuum brake booster just to get more line pressure. You get less required driver effort, for the same or better braking performance.

Disk brake calipers generally require more fluid displacement to operate effectively. ( and also more line pressure ) Than drum wheel cylinders do. Which is why you move up the a lager bore master cylinder.

Changing to a " worse " pedal ratio doesn't result in less line pressure. Your leg having " less leverage " doesn't really matter. When you've added power assisted brakes. That lets you achieve the same or greater line pressures. With less physical effort.

Race cars also have . . . Racing calipers, racing compound brake pads, racing disks, race compound tires, racing suspensions. It's not just a manual master cylinder that makes them stop just fine.

[theastronaut]

CPP says that 100 pounds of input force on the pedal with a 7:1 pedal ratio, 7" dual diaphragm booster running on 23" of vacuum, and a 1 1/8" MC bore will make 1197 psi in the lines- 704 psi from pedal ratio leverage/the master cylinder's piston area and 493 psi added from the booster's assist. Most hot rod engines don't make 23" of vacuum. If you drop the pedal ratio to 4:1 like most do in a boosted setup you'd lose line pressure with the same 100 lb input force because you've lost leverage. So in a real world setup you're not getting the full 1197 psi line pressure.

A 7/8" bore MC without a booster and a 7:1 pedal ratio will make 1166 psi with the same 100 pounds of pedal input force. So, why add a booster to end up with basically the same line pressure? It doesn't make sense when you could just add a smaller master cylinder and achieve the same line pressure.

If you have all air out of the system the pedal isn't going to drop too far with a smaller bore. Disc pads don't retract that far, and you should have working self adjusters in the rear drums so the shoes shouldn't retract far. My non-boosted pedal drops level with the gas pedal when braking, perfect for heel/toe downshifts. Its nowhere near the floor and maybe has 1-1.5" of stroke at most before building pressure. Once it builds pressure it stops dropping because there is no air in the system to compress. I have enough braking power to lock up the wheels at 80 mph, I don't need "more braking power". I already have more braking power than the tires have traction. More braking power for the same pedal input force would make the brakes harder to modulate and easier to lock up in an emergency stop situation. This is extremely important when you don't have ABS. An overboosted easy/light pedal is dangerous because it makes the wheels lock up too easily.

One of the common mods most people make when building a street car into a track car is to delete the booster and switch to a smaller bore MC for better pedal feel and easier modulation, especially on non-abs cars.

Businesses exist to sell parts, whether you need them or not. People have been led to believe that you need a brake booster to have effective brakes, but as the math shows it's just not true.