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Brake Pedal Sinking after replacing all brake fluid

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Slipalong

Member Since: 03 Nov 2018
Location: Sussex
Posts: 29

2006 Discovery 3 TDV6 SE Auto Bonatti Grey

Brake Pedal Sinking after replacing all brake fluid

Hi,

Car failed its MOT due to worn rear upper suspension bushes and corroded rear brake pipes. So new rear upper control arms fitter and all new pipes fitted. Due to the difficulty replacing the rear piped end to end I removed the exhaust and heat shield and joined the original steel with copper. While the pipes where cut, and I found a flaring tool man enough to flare the original steel pipes, all the old brake fluid drained out because I didn't wedge the peddle down

So had to refill the whole system using the brake bleed function on the Foxton Diagnostic unit. No leaks but when I press the pedal it applies the brakes but if I maintain normal pressure the pedal slowly sinks. Is it possible I have screwed the seal in the master cylinder? Obviously during the bleeding process the pedal travel the full extent which it never dose normally.

19th Oct 2019 11:54 am

jenseneverest

Member Since: 12 Jun 2017
Location: somewhere
Posts: 760

2005 Discovery 3 TDV6 Base 7 Seat Manual Zambezi Silver

the pedal sinking is still air in the system, start by bleeding from the longest pipe to the shortest.

19th Oct 2019 12:00 pm

Slipalong

Member Since: 03 Nov 2018
Location: Sussex
Posts: 29

I have read other articles which say air in the system gives a springy pedal (as the air compresses and expands) but a failed master cylinder seal causes pedal sink slowly and not bounce back. Happy to be convinced otherwise, but seems to make sense to me.

I thinking that there was a ridge in the bore in the master cylinder and that has knackered the seals as the piston moved way passed where it normally would do.

19th Oct 2019 12:22 pm

jenseneverest

Member Since: 12 Jun 2017
Location: somewhere
Posts: 760

Yes it is possible it is a master cylinder, like you say, but was it poor before you changed the pipes ?
Are you sure you did not get any debris into the system ??
If it were me and the only thing i had changed was the pipework and fluid i would try bleeding it more first
Especially as you said you let the system drain.
Did you bleed it in order like i said above the longest pipe first to the shortest ? NR - OR -NF - OF

19th Oct 2019 12:41 pm

Slipalong

Member Since: 03 Nov 2018
Location: Sussex
Posts: 29

Hi yes I used the brake bleed routine on my diagnostic unit. Not sure how good to was as I could only hear the motor in the ABS unit runing for 2 of the 4 wheels?? The order the diagnostic unit instructed me to open the nipples was RL FL RR FR. The RL nipple was leaking fluid around the threads, but I was using a pipe and bottle with a non-return valve in the pipe. If I was getting a springy peddle I would suspect air had been sucked back around the thread.

I have completed the bleed process twice and put 1.5L of fluid through the system.

19th Oct 2019 12:50 pm

Slipalong

Member Since: 03 Nov 2018
Location: Sussex
Posts: 29

Just in case anybody else reads this with the same problem. Yes the master cylinder was the problem. Think there may have been crud in the tank which got sucked through and damaged the seal.

30th Oct 2019 4:59 pm

aja4x4

Member Since: 14 Apr 2019
Location: Westbury
Posts: 2459

2005 Discovery 3 TDV6 SE Auto Zambezi Silver

You can also damage the seals if you bleed the brakes by pumping the pedal.
the seals wear the bore and can leave a ridge, if you push the pedal to the floor it forces the seal past the ridge and can damage the seal.

30th Oct 2019 7:00 pm

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