Brake pedal goes stiff under heavy braking, no ABS engagement?

[CrankyOldMan]

Basically, under some conditions the brake pedal goes rock hard at partial travel. At no point does the ABS kick in. Once I slow down and let completely off the pedal or accelerate again, it returns to normal operation. The freaky part is that it tends to happen when I'm pointed at a danger area, like a curb or light pole pylon. Has anyone else has experienced this, even with another vehicle?

Here's my thoughts on it. Not that they are correct, but it's the conclusions I've come to. For background: I have a Blitz S/C, custom intake runner w/ cone filter, 1ZZ TB, stock rotors and generic pads.

1. Vacuum lock condition on the brake booster. The brake vacuum line that goes just behind the throttle body might not be a vacuum rated hose, and/or the bend radius is just enough to pinch off under vacuum during rapid throttle release.

2. Too much wheel lift on rough terrain for the ABS controller to deal with it. It tends to happen when going over a section of the course with wavy or uneven pavement and/or loose gravel. Seems like the ABS is designed to deal with this, no?

3. Ground slopes away from the direction of travel. The other common condition is that the braking point is on or near the crest of a slight rise/fall. I'm not talking about the deliberate "jump" at Grattan Raceway, just the normal crown/drainage grading of a parking lot.

Anyone have any idea?

[ArmstrongRacing]

Is the check valve still in your brake booster line? And in the correct flow direction?

[CoryM]

Your guess of a blocked vacuum line is probably correct. It's possible to get some erratic pedal effort due to a failing booster, but that's usually minor. Silly question, but is your vacuum line connected before the blower? Otherwise if you are making boost your vacuum stuff will not be working well :biggrin: .

Abs is likely not doing anything because you aren't able to brake hard enough to cause lock-up.

Cheers.


Basically, under some conditions the brake pedal goes rock hard at partial travel. At no point does the ABS kick in. Once I slow down and let completely off the pedal or accelerate again, it returns to normal operation. The freaky part is that it tends to happen when I'm pointed at a danger area, like a curb or light pole pylon. Has anyone else has experienced this, even with another vehicle?

Here's my thoughts on it. Not that they are correct, but it's the conclusions I've come to. For background: I have a Blitz S/C, custom intake runner w/ cone filter, 1ZZ TB, stock rotors and generic pads.

1. Vacuum lock condition on the brake booster. The brake vacuum line that goes just behind the throttle body might not be a vacuum rated hose, and/or the bend radius is just enough to pinch off under vacuum during rapid throttle release.

2. Too much wheel lift on rough terrain for the ABS controller to deal with it. It tends to happen when going over a section of the course with wavy or uneven pavement and/or loose gravel. Seems like the ABS is designed to deal with this, no?

3. Ground slopes away from the direction of travel. The other common condition is that the braking point is on or near the crest of a slight rise/fall. I'm not talking about the deliberate "jump" at Grattan Raceway, just the normal crown/drainage grading of a parking lot.

Anyone have any idea?

[ilikerice]

If Cranky is autocrossing, yea, he is definitely getting in the ABS zone sometimes. Everyday driving, probably not. Says it happens at crowns on parking lots, so its probably an autocross event.

[CrankyOldMan]

Is the check valve still in your brake booster line? And in the correct flow direction?

If there's a check valve, it's not in the section that I replaced. The Blitz kit only extends the connection from the brake booster to the intake plenum, which is just a hose.

Your guess of a blocked vacuum line is probably correct. It's possible to get some erratic pedal effort due to a failing booster, but that's usually minor. Silly question, but is your vacuum line connected before the blower? Otherwise if you are making boost your vacuum stuff will not be working well :biggrin: .

Abs is likely not doing anything because you aren't able to brake hard enough to cause lock-up.

Yup, the vacuum line from the top of the engine block (next to the fuel rail) goes directly behind the TB.

If Cranky is autocrossing, yea, he is definitely getting in the ABS zone sometimes. Everyday driving, probably not. Says it happens at crowns on parking lots, so its probably an autocross event.

Apologies, I assumed posting in the track/racing/autoX forum implied it was under performance conditions. =) Yes, it's been in parking lots.

I've recently attended a car control clinic and one of the exercises involved getting into ABS on dry, flat pavement--aka straight line braking. Got into the ABS 3 of the 4 times, so it shouldn't be an issue with "leftover" boost getting into the brake booster on rapid transition from WOT to brake. That just leads me back to the downhill slope idea...

[CoryM]

Regarding the ABS: I am not sure what tires/brakes you run on, but I don't think I would be able to lock-up my NT01s w/o a brake booster on stock brakes. In some cases I can barely cause lock-up even with power assist. If you're bored, try disconnecting the booster, and see if you can cause lock-up ;)

In terms of testing, you can remove the hose and uses a vacuum tester to see if the hose collapses. I would also wiggle the hose and valve while the engine is running to see if you have a leak. The boosters are known to fail on these car, but they are pretty hard to test if they are intermittent. My booster has a small leak at times, but not enough for my cheap-ass to replace it yet.

Cheers.

[Ben_]

I have noticed when racing my yaris on Miller motorsports track in toellee Utah when going backwards at high speed (after a spin) the abs goes rock hard and stops then pads from contacting the rotors. Not sure if that's helpful but I figured I'd share