Sinking Brake Pedal

[JAS21]

The other day whilst waiting for my wife( a not uncommon thing) to finish whatever she was doing, I was sitting in my New (well newish now) D-Max, with the engine running and pressed, in boredom, the brake pedal continuously, it slowly went down more or less to the floor, maybe not quite to the floor... I was a bit surprised. The brakes however still work very very well.

So I took it to the dealer today they had a look, it was up in the air for a while with some mechanic underneath, apparently made some tiny adjustment which my wife unfortunately couldn't fully translate.

Basically they said it was normal and in fact they showed me their demo vehicle and that did the same. When the engine is not running the pedal seems reasonably firm, but there again it doesn't get any help. Of course in years gone by a happening like this may have generated dirty underwear!

I've had a look around the internet and it seems a not uncommon thing ... various people suggested what the problem might be, this leaking, that worn etc, BUT a number said its a feature on some vehicles, especially diesels.

So can any expert actually explain this sinking pedal pressure phenomenon. I think the dealer said something like it's a feature of the air (vacuum) brakes ... probably mis-understood....thanks

[Briggsy]

You are, in fact, driving a dual-control car and the person in the passenger seat, your driving sensei, is pressing the brake pedal as you are still inexperienced.

[bangsaenguy]

Sounds as if your brake lines contain air and need to be bled

[transam]

You have a problem, pedal down is a no no. Modern braking systems are complicated and need someone who understands you rides system.

[BSJ]

Agree with Trans Am. Modern vehicles with ABS generally need to be vacuum bleed following the correct sequence.

[JAS21]

You have a problem, pedal down is a no no. Modern braking systems are complicated and need someone who understands you rides system.

I'm inclined to agree, normally it's a big NO NO ... BUT .... my wife knows someone high up in Isuzu Tech Dept, she has just spoken to him....he will get the vehicle looked at tomorrow. I'll let you know the outcome.

[JAS21]

Had the engineers down to talk to me. It's pretty complicated but associated with the ABS and the electronic brake boost system. So this sinking pedal only occurs when you are not moving.

The braking system looks at how fast you are going, how hard you press the brake pedal and how fast you press the brake pedal and then it decides what to do ... well in layman terms anyway. I had noticed and so had my wife, by her comments, that going quickly for the brakes even when going reasonably slowly, some pratt on a motor bike cuttting in, you get quite a sharp braking action even when you really only need a little slowing down to miss him..

[BSJ]

Sounds like Isuzu have found a complex solution to.......a normal correctly functioning brake system.

[jbrain]

Sounds like Isuzu have found a complex solution to.......a normal correctly functioning brake system.

My Nisan has the same, when I am stationary with running engine and I repeatedly press the brake pedal it will go fairly deep.

While drivng it will never go deep.

I remember in an previous life with older cars it would just be the opposite. When repeatedly pressing the pedal it would build up pressure untill point that it would almost not move anymore.

I guess technology advances.

[transam]

Sounds like Isuzu have found a complex solution to.......a normal correctly functioning brake system.

My Nisan has the same, when I am stationary with running engine and I repeatedly press the brake pedal it will go fairly deep.

While drivng it will never go deep.

I remember in an previous life with older cars it would just be the opposite. When repeatedly pressing the pedal it would build up pressure untill point that it would almost not move anymore.

I guess technology advances.

Petrol engine brake servo is fed by vacuum from the inlet manifold, diesel has a mechanical pump.

[scotto]

Sounds a bit silly to me but i am not a mechanic?

the car actually decides how hard to brake?

I hope its not kikiat

[transam]

Sounds a bit silly to me but i am not a mechanic?

the car actually decides how hard to brake?

I hope its not kikiat

No. You have a problem to sort.

[jbrain]

Sounds like Isuzu have found a complex solution to.......a normal correctly functioning brake system.

My Nisan has the same, when I am stationary with running engine and I repeatedly press the brake pedal it will go fairly deep.

While drivng it will never go deep.

I remember in an previous life with older cars it would just be the opposite. When repeatedly pressing the pedal it would build up pressure untill point that it would almost not move anymore.

I guess technology advances.

Petrol engine brake servo is fed by vacuum from the inlet manifold, diesel has a mechanical pump.

I don't know what makes the difference, but I have a diesel pick up same as the OP I guess, so the mechanical system will have these symptoms and the vacuum system not ?

[scotto]

The braking system looks at how fast you are going, how hard you press the brake pedal and how fast you press the brake pedal and then it decides what to do ... well in layman terms anyway

As i said the car decides apparently

[JAS21]

Keep the opinions coming guys.................. especially you experts.

Actually this evening I pressed the brake pedal and the accelerator at the same time .... incase someone can't understand how this is done ... left foot on the brake and right foot on the accelerator ...it certainly slowed down quick and the pedal didn't go down very far and it felt absolutely solid.

So I'm happy that there isn't a problem ... and scotto, don't forget that the ABS decides when to apply the brakes and when not to ... you have no say in it unless you try and pump the brakes which defeates the object of having ABS of course.

As someone said earlier modern braking systems are complex, to me Isuzu have done something to make it stop quick and in a straight line, I suppose the ESC will help if you need to brake hard on a bend ... someone will know no doubt.....................

[Lickey]

UK Mot testers will sit in car with the engine ticking over, press the brake pedal, turn off engine, the pedal should go hard, even push back a bit,

If it gradually sinks down, there is a problem somewhere, Try this, it should put your mind at rest,

[Gary A]

When your car is equipped with ABS - Anti-lock braking system, it watches how fast each wheel is turning. When braking hard it will sense if one of the wheels stops turning and it will ease off that particular brake until it starts turning again. This prevents loss of control if two or all the wheels lock up. You can test this on a gravel or dirt road. When the system engages it will make noise and the brake pedal will pulsate. Your wheels will not lock up. You will be able to stop in a straight line with no sliding.