Why is it that when my car is turned-off, I can only press the breaks a couple of times before it stiffens up?
Power brakes get their power from engine vacuum. Under the hood behind the master cylinder is the brake booster. When you shut down the engine you loose vacuum and the brakes get stiffer. This is why it is very important that if your car ever stalls and you need to stop, to stop ASAP and do not pump the brakes. You will bleed out the vacuum and your car will be much harder to stop.
No Brake booster. When the engine is on, the brake booster uses the engine to create a vacuum to assist you in braking. When the engine is off, you no longer get this assistance from the brake booster. Try out a car from the 60's or early 70's before they started installing brake boosters in all vehicles. Big difference.
Hello, I feel I haven't heard an answer of my taste, so please hear mine: In your car you have a servoed brake system (except if you are having an old 911 Porsche which was unservoed for better feel :D). This is a pneumatic drum, which has a membrane in the center. This membrane is connected with the brake pedal. One of the sides of this membrane is connected to the inlet tract of your car. When the engine is runnig, this inlet tract has a pressure smaller than the atmospheric (also called vacuum). think of it as a suction force. This force is applying a base force to the brake pedal. It's set up so that when you aren't pressing the pedal, the servo drum itself hasn't got the power to apply actual braking. But when you press it, you don't have to press it so hard, as without this base force. Thus, pressing the brake pedal is easier. This drum can store this negative pressure, for a couple of braking actions, because when the engine goes off, your brake power assistance would go away instantly and you wouldn't be able to stop, or at least with great physical effort. And here comes the REAL answer: When you get into the car, engine does not run, normally you STILL have some negative pressure in your servo (which means that you HAVE the brake assistance) ! That's why it's easy to push it a couple of times! After a couple of braking, the servo drum depletes, and afterwards you can feel the real power needed to brake your car! When you don't belive me, try this: 'pump up' the brake pedal, so that it's hard to push. Now it's empty. Put your foot on the pedal (not too hard) and start your engine! You will feel, that after revving up, the brake pedal will go down, after the drum has built up it's vacuum. I hope I helped you (and others) out with this. Regards
The power brake booster uses engine vacuum. When you hit the brakes a bit of the vacuum is released. With the engine running the vacuum is replaced and you do not notice anything, With the engine off, you use all the vacuum in the booster when you pump the brakes, and there is nothing left to power your brakes!!
Modern Auto Brakes have what is call Vac assisted brakes that mean that the braking process is made Easier with enging Vac supplied to a Brake Booster. What you are doing when you depress the pedal with the moter off is Bleeding off all the stored Vac in the Booster The hard pedal after bleeding off is what your brake would feel like without Vac Assist.
