BRAKE BOOSTER KNEE ARRIVES TOO SOON 1

[MRSSPOCK]

My brake booster seems to pass all the usual checks as regards holding vacuum with no leaks, and a pressed pedal progressing further downwards, when the engine is started, however, it seems to reach the"knee" point on the input versus output forces graph too soon. I can only assume it is an internal fault that is letting the control valve reach its advanced limit too soon. Anybody heard of this? Pedal starts off behaving like a normal booster operational pedal, but turns really hard too soon, and no further booster advantage is available. The booster still has really good vacuum even after sitting ten minutes with no engine running. And it takes the usual three pedal depressions to exhaust the vacuum from the booster, once the engine is stopped.

 

[GregLocock]

Hmm. I once messed about with some washers to adjust the knee point when I was younger and cleverer and stupider. Is this an entirely conventional setup, ie big vacuum cylinder and no electronics?

You might have a problem with the proportioning valve if your car has one.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[MRSSPOCK]

Thanks. Its a european 7th generation accord.
I see some videos referring to the proportional valve, but I wonder is that just a manifold.
Does an ABS car still need a proportional valve?
I would have guessed all that control would be in the ABS unit?

I have a 2 tonne pancake load cell that might fit in where the pads go.
Might be interesting to see how well balanced the forces are at each wheel.
I did do a touch test after some moderate braking and all calipers felt about the same temperature, so they're all active, and none is sticking.

 

[BrianPetersen]

What prompted this investigation?

What changed?

Give us some relevant history of the vehicle, particularly the history of the brakes. New pads recently? Pads done without replacing rotors? Brakes gone metal-on-metal? Any sign of brake fluid leaks? Caliper pins well-lubricated and pads not seized from corrosion or lack of lubrication (or improper fitment)? Have there been any "events" ... someone forgot to release the parking brake and smoked the pads, someone drove down the side of a mountain while towing a trailer and smoked the pads or boiled the brake fluid, someone's got bad driving habits and is riding the brakes all the time and smoked the pads or boiled the brake fluid?

Glazed pads (usually due to overheating) can require excessive clamping force in order to do much, and that might be causing the required pressure to exceed what the booster can deliver. So my area of suspicion isn't necessarily that the brake booster is bad, but that something's requiring excessive brake pressure due to a problem with the pads or disks.

If pads were replaced ... were they replaced with original-equipment or good quality aftermarket, or with cheap rubbish?

Most vehicles with ABS don't have a proportioning valve, they use the ABS hardware to perform that function, but I don't know the specifics of that particular vehicle.

It is certainly possible for a brake booster to go bad, but as far as I know, they're not designed to be serviced, only replaced.

 

[GregLocock]

"Most vehicles with ABS don't have a proportioning valve," That has always bugged me. ABS only kicks in at fairly high g (I think) yet the brake balance at say 0.3g is important, especially when cornering. Perhaps it is one of those cases where it is good enough.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?