‘Bottoming out’ doesn’t necessarily mean the pedal is physically touching the floor, but does refer to the pedal going to it’s ultimate range of travel, usually limited to the piston in the master cylinder hitting the bottom of the cylinder bore. Either way, it is not good. The piston should not ever hit bottom. If it does, either the piston cannot build brake pressure or you have a leak. Since a leak has been eliminated as a possibility, a bad MC os the next suspect.
A test for a bad MC is to pump the brakes when the pedal bottoms out or close to it. If it firms up after 1 or 2 pumps, the MC is bad.
Thanks for the info. Thanks for clarifying what “bottoming out” means. I didn’t know the inner workings of the specifics you outlined, but it has felt low to me; especially during snow/ice breaking. Okay, so if the pedal firms up after a couple pumps, that means it’s bad. If it were actually good, what would I experience then? Pumping doesn’t result in any firming? I’m going to give that a look today. Thanks again.
Thanks for the complements.
No, a bad master cylinder does not trigger a brake warning light. The brake warning light responds via a float sensor to low fluid in the reservoir. Low fluid also means it’s time to check the pads, because as the pads wear, the fluid moves from the reservoir into the cylinders in the calipers and (in drum brakes) the “slave” cylinders. When the pads are worn beyond a certain point, the fluid will have dropped to a point to trigger the warning light.
The warning light’s circuit also has a parallel loop designed such that when the parking brake is on the sensor’s circuit is bypassed and the light lights, to let you know your parking brake is engaged.
The car’s computer has no way of comparing how far the pedal has traveled with any other parameter, such as line pressure, fluid movement, or anything else. It cannot detect fluid leaking past the pistons in the master cylinder’s chamber.
Busted’s quick test allows the MC to build pressure if marginal passage of the fluid past the MC piston’s seal. O-ring seats are designed to allow the O-ring to seal under pressure by getting squeezed into a corner, if you will. A seal at rest can leak, but once the pedal is pushed it begins to seal better and repeated pedal pushes can build pressure. It’s a good test, but it can be misleading of there should be air in the system. Air will compress, and the pumping will allow the air to compress and the pressure to be built up. In your case, I think the seal failure is a bit more substantial, and may even be dependent upon the piston’s position in the cylinder.
For your symptoms, I’m still betting you have a bad MC.
One more comment: you may or may not know it yet, but you, my friend, are a natural born engineer. If you haven’t chosen your life’s path yet, you may want to consider this as an option. You have the kind of relentless inquisitiveness and ability to visualize and comprehend that makes a great analyst, and an engineer is nothing but a relentless curious inquisitor with a good grounding in math and physics.
If you have a collapsing brake hose or a sticking brake caliper that can cause one wheel to lock and the ABS to reduce pressure to all the wheel to get the sticking one to turn.
One more thing to consider: Does the car have drum brakes in the rear (I know it has disks in the front)? Out-of-adjustment drum brakes can cause a low brake pedal and poor overall braking performance (don’t ask me how I know this).