poor brake pedal

[joohn1]

Hi all,

I have a 2005 Rab 4 the break pedal with out the engine running is solid but with it running it slowly moves to the floor when pressed, is this a ABS fault?

 

thanks Joohn1

[flash22]

It could be the booster or vacuum problem (or vacuum pump if it's a diesel)

A bad master cylinder can cause similar problems, And drop in the brake fluid level ??

[joohn1]

I dont think it's a bad master cylinder as with the engine off the pedal is solid and remains the same position for as long as you want to press it, I am not loosing fluid at all, but with the engine running the pedal slowly moves down to the floor in say 20 seconds, I can't understand how the oil is returning to the resovior.

[flash22]

It's the booster then, if you rev the engine does the pedal feel change

[joohn1]

I will try that tomorrow what is the fix if it is?

By booster do you mean servo?

[Wynford]

Hey John,

I get this too. Following this post hopefully for a definitive answer. 

[Sooty]

Hi,

I would suspect that the internal seals in the master cylinder are weak / leaking.

The servo when operating gives you more pressure onto the master cylinder. Hence why you can push the brake pedal to the floor.

[joohn1]

I have heard it could be the ABS leaking back

[flash22]

That would only happen if the master was bad, if it was a master issue if you pumped the brakes and pressed on the pedal you would feel the pedal sink slowly

brake booster aka brake servo, has a one way valve in the pipework if this is letting passed it will not hold vacuum

is it a diesel or petrol ?

[joohn1]

Hi again,

Before you spend a lot of cash please read this post.

I have just purchased a 2005 Rab 4 for my partner, when we arrived home there were some fault codes on the list relating to the VSC and TRC lights that came on. I took a good look at the running gear and documents supplied with the car, the last owners had spent a lot of cash on the breaks, there were new rear discs and pads fitted, so I decided first to fit new discs and pads to the front before I to look at the spongy pedal. I first removed the caliper it's held on by 2 bolts to a pair of brake guides these slide in and out easy as the pads wear down they keep the caliper central, one was completely seized - total commitment to move it with a large ring spanner, the rubber bellows had come away and allowed water in to rust it up. The replacement guide sleeves were only £7 a side, I completed both sides. 

The spongy brakes and large pedal movement are a result of the caliper being held back by the seizedsed guide at the top, the caliper would have distorted pressing on the lower half only and as they wore it needed more and more pedal to take up the slack probably throwing the ABS sensor out of stable and generating the fault codes and lights.

I took it out for a test run very good braking, I tried hitting them hard and getting them hot. All good and the pedal is now solid, I believe this was the cause of all the trouble, I cleared the fault codes and the car has now done a lot of work without any VSC or TRC lights appearing. 

It's worth checking that these guides are free and the cost of a disc was £19 not worth putting the grooved ones back on.

Joohn