Brake Pedal Sink...

[JustinF80]

Hi,

When you put pressure on the brake pedal of my MG ZT (CDTi) the pedal will slowly sink to the floor as you maintain the pressure on it, it is much more pronounced when the engine is running (and the servo is operating), when the engine is switched off it is much firmer.

Im hoping that it just due to old brake fluid that need to be changed!

Does anyone have any ideas on what this could be?

Justin

[Starman]

Hi Justin. I don't know exactly what causes it but I have the same thing. Also a faint whine which lasts a few seconds if you pump the air out or push quite hard.
I did read somewhere that it's the fluid seeping back over a valve or suchlike. But don't hold me to that.

Also do you sometimes get brake failure showing on the dash on start up?

[JustinF80]

Thats exactly what happens, the warning light comes on very intermittantly and only for a couple of seconds, the noise I get is a sucking/whining sound, the strange thing is that there is no fluid loss and some times the noise goes away for a couple of days

I was going to see if a full system bleed and fluid renewal was going to solve it.

Justin

[T-Cut]

I call it the Sinking Pedal Phenomenon (SPP). From my readings, the effect is very commonly reported across a wide range of brands, including Jaguars. Nobody has yet explained it (to my satisfaction anyway), offering explanatios relating to the evacuation of the servo causing it. Personally, I don't go for a simple mechanical explanation. I think it's a system control effect causing the ABS to recycle hydraulic fluid under the conditions prevailing when you do the test. The control system sees the brake pressure applied fully when the engine's running but the wheel's aren't turning. The SPP only happens when the engine's running. Of course when the wheels are turning, SPP doesn't happen and you get normal braking with no pedal sink.

You can find several discussions by searching for "SPP" (include the quotes).

TC

[Starman]

Wowzers TC. That's quite a phenonenom! At least it's nothing to worry about and a relief to hear. I've been working on the idea that someday I may be replacing components in the braking system or have brake failure creeping in.

Cheers for such an in depth explanation.

[David Lawrence]

Originally Posted by Starman

Wowzers TC. That's quite a phenonenom! At least it's nothing to worry about and a relief to hear. I've been working on the idea that someday I may be replacing components in the braking system or have brake failure creeping in.

Cheers for such an in depth explanation.

Yes I found a web site explaining it also while I was searching for some parts. It said that it is most common on diesels because they use a pump instead of engine vacuum, and that the cure to stop the pedal sinking, is to stop pressing it.
Seemed a rather sarcastic response, but as the car doesn't start rolling when you back off the pressure then I suppose it was a reasonable one.

Read here:
http://www.brakesint.co.uk/technical.html#techNote202


What I'm more interested in knowing is more about something i read on here from a guy who did the brake bleeding using a T4. He said that using a T4 he got loads more air out of the system than by using normal bleeding. But I didnt hear any more about it.

[JustinF80]

I have read the same about the T4 process, my local garage is has a T4 and the brake fluid is well overdue for replacement so Im going to get a quote for the job - im just concerned that they have never done it via the T4 before!

I will update when the work is done

Thanks T-Cut for the explanation - the only problem is that once you know its there it drives up mad!!

Justin

[T-Cut]

I'm not saying the above IS the explanation, I'm saying I believe it's related in some way to the control systems. Exactly what causes the fluid to bleed from the master cylinder has yet to be explained. The diesel vacuum system versus the petrol vacuum system is a red herring, as most explanations are. You can be sure it's nothing to do with how the servo is evacuated.

The use of a T4 to bleed the brakes relates to the ABS unit. The T4 can operate the ABS pump to allow fluid to be purged from the module. However, I honestly doubt that a T4 session is essential in the routine bleeding of 75/ZT brakes. There may be rare circumstances where air has got into the ABS module but it won't happen under normal use. Complete drainage of the fluid reservoir would cause it, but then the whole system would be compromised anyway.

TC

[JustinF80]

They were aware of the function on the T4 but as you also said they did'nt think it nessarcary. All booked in for a full service as the car is overdue!

Justin