Brake fade

[ian6]

I own a 2007/8 diesel which gave me a little scare yesterday, leaving the motorway and approaching a rounderbout, slowly braking with steady pressure on the pedal i became aware  the brakes appeared to be fading away, and no extra pedal pressure seemed to effect any further stopping. This problem only appears from speed over a distance, normal day to day driving is not affected, leading me to believe that this is either a vacuum pump or servo problem. The car has covered 95000 mls, new pads 1000 mls ago. Wondering if any of you good people can shed any light on this, will I benefit from a change of fluid/bleed etc., or has anyone had a similar experience. Your views and assistance would be appreciated. Regards Ian

[Old Peter]

Hi Ian,

 

I have not had this experience with a modern car, but I did with a Triumph Herald in 1966.  I put new brake fluid in and bled the brakes, and it worked OK.

 

Peter.

[Raistlin]

Ian,

 

You are almost certainly correct in your initial diagnosis.

 

The diesel engine doesn't produce sufficient vacuum to operate the servo so there is a vacuum pump.

 

The symptom you experienced is "hard pedal" and occurs when the vacuum has been exhausted and cannot be replenished rapidly enough.

 

Although it seems like the brakes have failed, they are still operative but without servo assistance and the S Type being a big heavy car, direct braking requires massive amounts of force on the brake pedal to be even marginally effective.

 

The servo on the diesel suffers from internal contamination leading to degradation of the seals so I'd be considering this before thinking about the vacuum pump which is anecdotally at least, more reliable.

 

The servos cost ridiculous money at a dealership and are not, as far as I'm aware, available as a pattern part. in the unlikely event that it should prove to be the vacuum pump, they are even more expensive than the servo.

 

They do come up on fleabay and in specialist breakers regularly though but you HAVE to get the correct part number because, even though they may look the same at first glance, they are not.

 

If you lift the felt cover on the driver's side bulkhead under the bonnet you will be able to see the servo and a torch will let you make out the part number as the label will be directly facing you.

 

As you might have guessed, i've been through this before... got the t-shirt as it were

 

I found the car reasonably safe to drive for the most part while I waited for my replacement servo but it requires a bit of planning with regards to when to brake. Swift and decisive braking paying dividends rather than using finesse, as that will exhaust the vacuum in the servo more rapidly.

 

As you have discovered, a few minutes later, there is sufficient vacuum again.

 

Don't be afraid to stamp brutally on the brake pedal if the situation requires it. You are still connected to the brakes, but without the benefit of assistance.

 

If I can be of any further help please let me know.

[O.T.H]

Hi Ian

I am in total agreement with Paul and I think he gave you a perfect answer

to your problem and will just add check all pipes on the sevo .??

 

Frank

[ian6]

Cheers guys, think in the first instance going to get the fluid changed and the system bled, just for my own peace of mind, then move on from there, anyone know the operating pressure of the servo/vacuum system?, think I read somewhere its round about -0.8 bar, don't like getting something tested without prior knowledge of some kind. Any and all info gratefully received. Regards Ian

[Raistlin]

Should be at least 18 mmHg, whatever that is in Bar.

[ian6]

Thanks Paul, that equates to 0.024 bar so I don't know what book I was reading, thanks again for the info all, will update when solution is found. REGARDS Ian