Braking problems

[Si B 0]

The brakes on my G60 have been getting worse for a few months now (probably something to do with a few too many track days :D ) and I have therefore just rebuilt the calipers, put some new pads on and bled the system... however the is something odd going on with the pedal:

 

With the engine off the pedal feels nice and firm with very little play or travel and exactly as I would expect it to. However, as soon as I start the engine it goes very soft and there is lots of travel before any real braking happens. I am pretty sure that there is no air in the system having bled the brakes a couple of times just to be sure, plus the pedal is good and firm with the engine off. The brakes also work very well and stop the car as I would expect albeit after miles of pedal travel. I'm guessing that the problem has something to do with the servo as this is the only part of the braking system that is linked to the engine but was hoping someone could suggest what might be wrong with it or even just confirm my diagnosis.

 

In hidsight, the problem probably existed before I overhauled the brakes but with threadbare pads and tired fluid I never really noticed it as a specific problem.

 

Any suggestions??????

[corradophil 3]

Try bleading it again, I had similar problems and after leaving it a few days a whole load of air came out. I bled a good 2 litres of fluid through before I got the air out.

Are you clamping the bias regulator whilst bleading?

[Si B 0]

No, I'm not clamping the bias valve when I'm bleeding, does this make a difference? I assume you mean clamping the bias valve shut when bleeding the fronts, and fully open for the rears, is this correct.

[corradophil 3]

I used an easy bleed, and clamped the bias valve the whole time, although it will not make any improvement to bleeding the fronts. Definately clamp it for the rear.

[2cc 0]

There will be a 'softer' feel with the engine running because of the vacuum assist. The question is, whether the pedal travel is excessive - ie greater than the designed limits.

Have you checked to make sure you have not introduced air into the master cyl? This is possible and there is a bleed nipple on some of them to get rid of it

[Si B 0]

The pedal travel is definately excessive. I am pretty sure that I have not introduced any air into the master cylinder as I am always careful to keep the level well above the min mark when bleeding.... but you never know!

 

I think I will start from scratch and drain and reflush the system at the weekend. It can't do any harm and if it doesn't sort the problem at least it eliminates something from the equation.

[NickRStorm 0]

My VR6 had exactly the same symptoms when the brake lines were replaced & it drove my mechanic mad trying to sort it. I told him to ring Vince @ Stealth & he was told to bleed the master cylinder LAST & suddenly the brakes had proper feel with the engine running.

[Kevin Bacon 5]

It's worth disconnecting a front ABS sensor before bleeding and locking the brakes up a few times. This will get all the old cruddy fluid out of the ABS pump and into the main system ready to be bled out.

 

And yep, do the MC last. The best way seems to be front calipers, rear calipers, ABS pump, then MC.

 

The rear calipers are responsible for long pedal travel too if not setup properly when fitted. I've found using MK4 rear calipers with MK4 rear pads (2mm thicker than Corrado ones) really does sort out the sloppy pedal. Good braking parts helps too, and Brembo discs and Pagid Fast road pads all round seems to be a brilliant and cost effect combination.