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Re: Left foot braking with simultaneous throttle (archive)

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Posted by Randy Walters on December 19, 2001 at 09:21:41:

In Reply to: Left foot braking with simultaneous throttle posted by John on December 18, 2001 at 16:20:23:

When I warm up the (race) pads, I stay on the
gas and left foot brake. My pedal gets firm
and high (as if vacuum lost or booster out).
This is from stone cold. So no way its pad fade
(which also has a high pedal) and fluid fade
has a low pedal, so thats out. Discs, pads etc. new.
Calpers rebuilt. New fluid, everything bled.

After about 2-3 max 5 times of successive left foot
(significant) braking (with right foot on throttle),
I loose boost. So, is this normal or a defect?
BMW technician I know says its normal. The purpose
of the membrane-valve in brake hose going to booster
is to provide enough vacuum, but after several brake
applications, its normal that the vacuum is lost.
Is this correct?
Thx. for any ideas

What you're experiencing during brakepad warmup
is normal. I warm up my brakes the same way at
the autocross and i always loose boost and the
pedal gets rock hard until i get off the gas,
then it comes back. It does this on my 89 325is,
on Gustaves M3, and on my previous 85 GTi
autocross car. It has something to do with
the booster not getting the required vacuum
until the throttle is closed, but i don't know
the exact reason why a booster acts like this.
The pedal kinda locks up after a few LFB
applications on the way to pregrid, but as
soon as i get off the gas the vacuum returns
and the brake pedal sinks and engages the brakes.
Try it again but notice how when you let off the
gas your brake pedal comes back.

I've had the exact same thing happen when my
stalled GTi was towed back to the pits with a
tow-rope, after a few brake applications i
suddenly had a rock hard pedal because all the
vacuum in the booster had been used up by the
previous few brake applications while being towed
by the pickup truck. I jammed it in gear and the turning of the engine replenished the
boost just in time to get some brakepedal back.
Almost rear-ended the tow vehicle !

89 325is

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