Brake pedal kickback

[Telco]

The Jag brake thing improving the feel got me to thinking... my 2003 LS V8 will stop OK, but right before it comes to a complete stop the brake pedal will push back on me and it'll try to not stop. Any idea what causes this?

 

[TooManyToys]

Could be getting a inconsistent signal from on the the wheel sensors, making the ABS think that specific tire is starting to skid.

 

[Kumba]

I had that problem on my '95 SHO and it was from broken tone rings for the ABS. That's all I got to offer you on the problem.

 

[joegr]

I've seen this on another car (Grand Marquis). In that case, it was caused by an incorrectly adjusted brake pedal position sensor switch. The result was that when the ABS kicked in, it would raise the pedal position and make it very difficult to stop. It doesn't sound like this is exactly your issue though.

 

[Telco]

Weird. Several possibilities. Guess when I take it in for a trans flush I'll just get a full diag on the entire car to get an idea of exactly what kind of problems I've bought here. Used car, someone else's problems.

On this, I don't feel or hear the ABS kick in, and it happens when I am driving it easy.

Thanks folks.

 

[M4rk]

I am possibly having the same problem on my 2003 V8, except it doesn't feel like the pedal kicking back, but rather like brake pressure diminishing right before coming to a stop, and the car pushing itself forward with more force than usual. Happened about twice today (warm brakes), and twice before (cold brakes).. getting a little scary.

 

[04_Sport_LS]

My first thought was vacuum booster, (vacuum leak, or loss of vacuum). Then I remembered that this is a newer car that is controlled by 15 different computer modules. On a whim, went and looked at the vacuum booster on my '04, and there is something screwed into the front of the booster with a wiring harness plugged into it. Could the vacuum assist somehow be electronically regulated on this vehicle???? I noticed that the one time I had to make a very sudden panic stop, the brake pedal actually dropped drastically away from my foot, and then came back slightly.

 

[joegr]

My first thought was vacuum booster, (vacuum leak, or loss of vacuum). Then I remembered that this is a newer car that is controlled by 15 different computer modules. On a whim, went and looked at the vacuum booster on my '04, and there is something screwed into the front of the booster with a wiring harness plugged into it. Could the vacuum assist somehow be electronically regulated on this vehicle???? I noticed that the one time I had to make a very sudden panic stop, the brake pedal actually dropped drastically away from my foot, and then came back slightly.

No.
It's electronic braking assist. It stomps on the brake pedal for you if it senses a panic stop. It can't reduce braking force.

 

[TooManyToys]

Some accident investigation studies showed that during panic stops people pulled back slightly on pedal effort when they felt the ABS pulsation in the brake pedal, reducing the hydraulic pressure to less then optimum tire skid pressures.

The ABS software detects the deceleration value, the brake pedal application rate and a few other inputs to make a decision to override the hydraulic pressure currently applied by the driver and uses the ABS pump to go to a higher psi insuring all the wheels will be in ABS skid mode. When the ABS module detects the driver has come off the pedal more in an attempt to release the brakes, it stops the activity and goes back to the straight, driver applied value (no ABS assist).

 

[M4rk]

I forgot to mention, while experiencing brake problems yesterday, I quickly pulled over to check the fluid level, and the rubber diaphragm inside the master cylinder cap was fully extended out

 

[joegr]

I forgot to mention, while experiencing brake problems yesterday, I quickly pulled over to check the fluid level, and the rubber diaphragm inside the master cylinder cap was fully extended out

That's okay. It's supposed to do that as the fluid level falls. Assuming that you don't have any leaks, the level falls as the brake pads wear away. This is likely an indication that you need or will soon need to replace the brake pads and maybe the rotors. I assume that the fluid level isn't below the min mark. If it is, you should have a low brake fluid level light on.

 

[02LincLS]

I have noticed that the transmission downshifting can be felt as well. Put your car in neutral next time before you press the brakes to come to a stop. It seems to stop much faster in the last few feet when in neutral. Its almost as if the computer raises engine RPM's slightly to drop down a gear easier when you're in gear.

 

[M4rk]

That's okay. It's supposed to do that as the fluid level falls. Assuming that you don't have any leaks, the level falls as the brake pads wear away. This is likely an indication that you need or will soon need to replace the brake pads and maybe the rotors. I assume that the fluid level isn't below the min mark. If it is, you should have a low brake fluid level light on.

Fluid is right at MAX. Replaced all rotors and pads last year, all of which have plenty of life left. I pressure bled my brakes a few weeks back however, with Prestone DOT 4 fluid. That's about when the problem started IIRC..

I did some extensive reading of the service manual during my lunch break, which outlined a few tests - I will go through them once I get home (if I make it in one piece :shifty.

 

[02LincLS]

Its possible you may have some water in the system and the fluid is boiling, giving you the spongy feel. Or simply still some trapped air.

After much research, I went with Valvoline Synthetic Dot 3 & 4 fluid when I flushed mine. It was a good balance of dry and wet boiling points. It can be misleading to buy a fluid based solely on dot number. You really need to look at each individual brand of fluid and find their specific wet and dry boiling points. Make sure the wet rating is with 3.7% water by volume.

 

[M4rk]

Its possible you may have some water in the system and the fluid is boiling, giving you the spongy feel. Or simply still some trapped air.

After much research, I went with Valvoline Synthetic Dot 3 & 4 fluid when I flushed mine. It was a good balance of dry and wet boiling points. It can be misleading to buy a fluid based solely on dot number. You really need to look at each individual brand of fluid and find their specific wet and dry boiling points. Make sure the wet rating is with 3.7% water by volume.

Not a spongy feel, just an intermittent loss of braking power for split second when almost fully stopped, or at least that's how I perceive it. Almost feels like the car lurching forward as if the IAC was bad, but Gen IIs don't have those.

Prestone DOT 4 Synthetic (all that was available):
Minimum Dry Boiling Point 500F (260C).
Minimum Wet Boiling Point 311F (155C).

 

[TooManyToys]

Pull your ABS fuse(s) and see if it still does it.

 

[joegr]

Not a spongy feel, just an intermittent loss of braking power for split second when almost fully stopped, or at least that's how I perceive it. Almost feels like the car lurching forward as if the IAC was bad, but Gen IIs don't have those.

Prestone DOT 4 Synthetic (all that was available):
Minimum Dry Boiling Point 500F (260C).
Minimum Wet Boiling Point 311F (155C).

A slight jump forward at low speeds during mild braking seems to be normal (sometimes). It's the transmission shifting. I get it sometimes when pulling into my driveway.

 

[M4rk]

A slight jump forward at low speeds during mild braking seems to be normal (sometimes). It's the transmission shifting. I get it sometimes when pulling into my driveway.

Yep, I felt the gear change, and noticed the RPMs climb ever so slightly to around 1100. My brakes work perfectly fine otherwise, so I will let it be, but I am still puzzled by the gasket's behaviour. The old (original) one never did this.

Thanks Joe!

 

[Telco]

A slight jump forward at low speeds during mild braking seems to be normal (sometimes). It's the transmission shifting. I get it sometimes when pulling into my driveway.

That's exactly what it feels like. Good deal. Going to get the trans serviced in the next week or two (would have already, but I just had a bout of gut bother, found out it's a bad gall bladder, going for surgery next week) and they may replace that synchro pack thing you guys were talking about. I'll advise what happens on it after that.

 

[joegr]

... found out it's a bad gall bladder, going for surgery next week) ...

The good news is that they now just make three or four small holes in you to remove it.
The bad news is that the gas they inflate you with does tissue damage and your shoulders will hurt for a few days. (Actually, it's your diaphragm that hurts, but the pain feels like its coming from the shoulders.)