Just as a novice but I have driven some miles, I have only had one brake booster go bad. It was on my 59 and the symptom was the pedal would stick down and you’d have to pry it up with your foot. A $10 used one took care of it. I have had several master cyl though and the symptom was the pedal going to the floor and sometimes could be pumped up.
Just replace the dang thing. What can it cost? See a dealer if you have to.
Thanks, Bing. I’m willing to pay a pro so I won’t have to learn how to bench bleed the MC or how to bleed the entire brake system, and so I won’t have to mess with brake fluid. (This would be an opportunity to do the regular brake fluid replacement, as mentioned way back in this topic.) I just need to get a reliable diagnosis.
Does the hissing sound stop once the brake pedal bottoms out at the end of its travel, or does it continue hissing until you release the brake pedal???
Engine off:
Depress the brake pedal 4–5 times (you should have about 3 pedal pumps before using up all the vacuum assist) and hold the pedal down—do not release it…
While continuing to hold the brake pedal, start the vehicle… Watch/feel what the pedal does…
On a properly functioning vacuum booster, when the engine starts and vacuum is applied to the booster, the brake pedal should drop some under your foot… That’s the classic vacuum-booster functional test…
I have heard this complaint many times, even with new cars. I doubt you are really pressing the brake pedal to the floor. If the pedal was “to the floor”, the brake pedal would be compressing the carpet, and the brake warning light would be on.
Try applying the brake that hard while driving 25 mph; you will understand why that is a flawed assessment. You are applying the brake way too hard and compressing the brake booster.
If there is space under the brake pedal for the toes on your left foot, the pedal is not to the floor.
You have been driving this car for how many years without a brake pedal complaint? And now you found that the brake pedal has more travel when the vehicle is at rest.
You mechanic drove the vehicle, he is not guessing.
The pedal slowly sinking while you hold pressure is worth getting checked, as it can point to a master cylinder or hydraulic issue. The change in braking feel near a stop could have several causes too. I’d have the brake system inspected soon rather than keep driving and guessing.
The person with this problem did have a shop look at it.
I finally got enough time to do some of the recommended tests and to compose this long answer.
Responding to Nevada_545:
I have heard this [pedal to the floor] complaint many times, even with new cars. I doubt you are really pressing the brake pedal to the floor. If the pedal was “to the floor”, the brake pedal would be compressing the carpet, and the brake warning light would be on.*
I should have said that the pedal goes down until something reaches a hard stop. (The pedal goes pretty far down. “To the floor” was an inappropriate colloquialism.)
Try applying the brake that hard while driving 25 mph; you will understand why that is a flawed assessment. You are applying the brake way too hard and compressing the brake booster.
Thanks for the new suggestion. I did this, and the pedal was firm as the car came to a stop (even a little ABS action). After the car stopped, the pedal (as before) moved slowly downward.
You have been driving this car for how many years without a brake pedal complaint? And now you found that the brake pedal has more travel when the vehicle is at rest.
That’s the whole point. Driving this car – almost exclusively – for 19 years, I did not notice any soft-brake-pedal problems. Then I drove a different car that had much sharper brake pedal action (less force, less travel). So I wondered, “Could it be that my car’s braking has slowly deteriorated, and I have not noticed it because the brakes still work OK?” That’s what prompted me to start experimenting and to ask my original question: Do the brakes need fixed?
Quoting art1966: I think that’s what everybody here (including myself) is guessing.
“Guessing” was a really bad word choice. I should have said “using our knowledge and experience to predict what we will find when we test”
You mechanic drove the vehicle, he is not guessing.
That’s the other whole point. My Trusted Independent Mechanic says I’m OK. Some contributors here are doubtful.
Responding to davesmopar:
Does the hissing sound stop once the brake pedal bottoms out at the end of its travel, or does it continue hissing until you release the brake pedal???
Stops when the pedal bottoms out.
Engine off:
Depress the brake pedal 4–5 times (you should have about 3 pedal pumps before using up all the vacuum assist) and hold the pedal down—do not release it…
Takes only 1-1/2 strokes before hissing stops (and pedal stiffens).
While continuing to hold the brake pedal, start the vehicle… Watch/feel what the pedal does…
On a properly functioning vacuum booster, when the engine starts and vacuum is applied to the booster, the brake pedal should drop some under your foot… That’s the classic vacuum-booster functional test
Just like you said, except the pedal bottoms out (instead of dropping “some”).
The problem is how the brake actions were being describe. ‘Bottoms out’ to me is nearly the same as “to the floor”. “Soft pedal” to me is air in the brake fluid. Another thing you posted is “pedal slowly sinks to the floor”, or words to that nature, to me, that indicates either a leaking master cylinder or wheels cylinder. If fact, that was a recommended check when purchasing a used car. Yes, drum brakes not correctly adjusted, can cause a low pedal. When I had drums I made it a habit to hit the brake pedal hard when backing out of my driveway. Seemed that’s all it took to keep them adjusted.
Not trying to be too critical. This is not uncommon. Much like a person stating their car won’t crank, when in reality the car cranks fine but will not start.
Just for the heck of it, tried both my car and truck. Engines off, I don’t like to start them unless I am going to drive them. Neither vehicle has drums. 2013 truck, around 72000 miles, pads have been replaced, rotors resurface about 20,000 miles ago, pedal hard, does not sink under pressure. 2020 car, 32000 miles, no brake work yet, pedal hard, does not sink.
Update: Tried with car running, again pedal does not sink.
Now you need to do the clamp off all the brake hoses and release one at a time thing to see what it does (as I posted about prior), that is what will tell you the most about your brake pedal slowly sinking issue, or where to look next basically and can eliminate everything below the clamped area…
Can still have other issues like internal bleed off in the ABS modulator (if you have ABS), but that is for another day…
Make sure to use proper clamps or make your own, if you don’t know what you are doing you can damage the (rubber) brake hose… Any shop that does brake repairs should easily be able to do this, if they don’t know or understand how to do this test, then it is time to find a new brake shop that does…
The hard part for us Pros and the skilled DIYers is we can only go by what you are describing since we can not test drive to confirm your issue, we might say it is normal if we were to feel the pedal ourselves, or we might all agree with your findings, but from being behind a keyboard it makes it much harder to help anyone in your situation…
What was this other car? Every make and model of car can have a different brake pedal feel depending on the design and operation of the system and on many cars these days the brake pedal isn’t even directly connected to the hydraulic system that actuates the calipers–a “brake by wire” system.