Brake Booster Function

jlfents · 15 Feb

Hi all,

So how exactly is the functionality of the brake booster applied?

I know the vacuum from engine creates a pressure differential across the booster diaphragm creating a force, which is where the booster generates its assistance from. I am confused about how this assistance is applied.

Force on brake pedal is multiplied by pedal ratio. That multiplied force goes into the booster. Now, I don't believe the booster then applies its full potential of assistance all at once. Is there a valve inside the booster which regulates how much assistance is provided? I.e., and really the question I need answering is, does the booster multiply the force going into the booster until the booster can provide all of the assistance it can provide?

If this is right, does anyone have any data on what the multiplication factors are for the Volvo boosters? I'm mainly interested in 140.

jamesinc · 15 Feb

Inside the booster there is a diaphragm. On one side (the side that faces the brake master) you have engine vacuum. On the either, you have either engine vacuum (when the brake pedal is travelling upwards), or ambient air (when the brake pedal is travelling downwards).

When you press on the pedal, you press a poppet valve that isolates the two halves of the diaphragm from each other, and at the same time allows air in via an intake (it's what the black plastic piece around the pushrod on the cabin side is covering).

When the pedal starts moving upwards as you release the brakes, the poppet valve isolates the booster from ambient air and joins the two halves of the diaphragm, allowing the air in the booster to be evacuated into the engine intake.

Applied force would be area of the diaphragm multiplied by the pressure differential. It would be a lot, I reckon well over 100kg for most passenger cars.

I think the key concept behind the booster is that it provides assistance as a function of the brake pedal's acceleration, and how hard you are pressing on the pedal rather than as a function of the absolute position of the pedal in its range of motion. It doesn't multiply braking force quite the same way lever ratios do.

jlfents · 16 Feb

Ok. That answers my question . Thanks James

It doesn't make it easy to predict how the booster is behaving in terms of numbers though lol.

Just for discussion sake, my 8 inch single diaphragm booster can provide up to 2000N of force at 18" vacuum, equivalent to a ~200kg weight. As you apply the brakes, I'm having a hard time understanding how the booster can regulate that much force without the brakes feeling jerky. Does this poppet valve regulate how much vacuum the booster sees (maybe airflow through the booster is a better description?) depending on how hard or sudden the brakes are applied?