Brake boost lacking under left foot braking

[MrChris]

When left foot braking, if you touch the accelerator at all does the vacuum boost to the brake pedal get "depleted"? I've been noticing this recently, where if I left foot brake, happen to touch the throttle, the brake pedal goes quite stiff and braking performance is reduced. The brakes seem to come back fine when I get back on the throttle. Could I have a leak somewhere? Or is this normal (it doesn't feel normal).

[Ardent]

I don't know.

But my 2p says

Brake pedal (safety) over rules accelerator (fun) pedal.

[Petrus]

When left foot braking, if you touch the accelerator at all does the vacuum boost to the brake pedal get "depleted"? I've been noticing this recently, where if I left foot brake, happen to touch the throttle, the brake pedal goes quite stiff and braking performance is reduced. The brakes seem to come back fine when I get back on the throttle. Could I have a leak somewhere? Or is this normal (it doesn't feel normal).

You could check the one way valve on the booster which maintains tbe vaccuum.
Apart form that the volume of the vacuum boost is limited and yes it will loose pressure difference. The system is designed on the assumption that the loud pedal is lifted when the brakes are applied.

On the old DAF variomatics I added an extra reservoir to have an intermediate ´sports´ function available ánd to have ´more´ vacuum (weird concept; adding an extra reservoir of nothing      ) to facilitate the left foot braking.

Anyway, check the one way valve and if that is ok, you can very simply mod the system with an extra ´buffer´.

[Alex Knight]

When left foot braking, if you touch the accelerator at all does the vacuum boost to the brake pedal get "depleted"? I've been noticing this recently, where if I left foot brake, happen to touch the throttle, the brake pedal goes quite stiff and braking performance is reduced. The brakes seem to come back fine when I get back on the throttle. Could I have a leak somewhere? Or is this normal (it doesn't feel normal).

This is normal behaviour for any car.

Strictly speaking, you should never be on the brake AND throttle at the same time. You are taught this for your ARDS licence.

You can do it, but only left foot brake gently whilst on throttle to trim your line.

[MrChris]

This is normal behaviour for any car.

Strictly speaking, you should never be on the brake AND throttle at the same time. You are taught this for your ARDS licence.

You can do it, but only left foot brake gently whilst on throttle to trim your line.

Okay makes sense thanks. Yeah I never intended to be on both at the same time 

I'm finding the more I drive/track day the car, the more in tune with it I am - which is kind of an obvious thing to say really - but the main thing I'm noticing is how the brakes could sometimes be amazing and sometimes drop off in performance. However, people who don't track the car wouldn't notice a difference but I definitely do. This got me on to experimenting with left foot braking and testing the brakes out in different situations, but I also suspect my brakes could do with a bit of a service/cleaning and bleeding to keep them in good shape.

[Alex Knight]

Very high quality pads and brake fluid are critical to on track performance.

[AdamR28]

Normal.

Brake servo is activated by engine vacuum created during overrun. Throttle on = no vacuum = no servo. Simples.

No harm done, just be aware of it.

[MrChris]

Very high quality pads and brake fluid are critical to on track performance.

Running Apec pads, as recommended by @AdamR28 and they are surprisingly effective. Also running RBF600 fluid with regular changes.

Normal.

Brake servo is activated by engine vacuum created during overrun. Throttle on = no vacuum = no servo. Simples.

No harm done, just be aware of it.

Good to know thanks. At least I know the brakes are solid, just need a bleed again.

[shnazzle]

Kind of miss my car
All of these escapades are definitely bringing in a feeling of regret.

I'm glad it's still being ragged around a track.

One day I'll be back

[MrChris]

Kind of miss my car
All of these escapades are definitely bringing in a feeling of regret.

I'm glad it's still being ragged around a track.

One day I'll be back

It's certainly getting put through it's paces. When the summer hits properly I'll check it for rust again but since I've done the work on it, its become a good work horse