Brakes

martin c brown · Jan 22, 2019

Hi All
I have a 2004 discovery I am having problems with the brake bleeding, I have bled the
brakes over and over but can not get them to hold up.
Every time I put my foot on the pedal it goes down to the floor. I can not see any air
bubbles when bleeding I have read this forum for advice, but still no joy.
The brake pedal will pump up for a short time after pumping three or four times, I have started at the nearest and worked my way back to the furthest and vice versa but still no air can be seen in the systems.
Any help with this would very grateful.
Thanks Martin

discool · Jan 22, 2019

Is this just a fluid replacement? So simple and straightforward, or after replacing parts possibly allowing air into the abs matrix, then not so straightforward

sierrafery · Jan 22, 2019

Replace the master cylinder

martin c brown · Jan 22, 2019

Hi, I have replaced the rear brake discs and callipers and I have replaced the brake fluid as well

brianconwy · Jan 22, 2019

Wrap some PTFE tape around the threads of the bleed nipples.

Si Click · Jan 22, 2019

Sounds like a dodgy master cylinder, but if not then could you have introduced some air to the ABS Control Module?
If so you might want to try a pressure bleed using a diagnostic tool.

martin c brown · Jan 23, 2019

Thanks guys diagnostic tool.i think

sierrafery · Jan 23, 2019

As long as the system was well bled with pedal the pedal should work well cos it has it's own circuit , power bleed with tester is necessary to get rid of the air trapped within the modulator in the return pump's circuit(OV and shuttle valves) and would affect only active braking modes when the electronically controlled pump is activated(TC, HDC, EBD)... so IMO if the pedal gets spongy after serious pedal bleed there's a leak somewhere or the master cylinder is gone ... the worst scenario is to be stuck OVs in the modulator and the pressure to drop there and return to the master cylinder but that's rare and the mster cylinder should be ruled out before suspecting that cos it's more common

martin c brown · Jan 23, 2019

Meny thanks for that i think i will look at air trapped within the modulator in the return pump's circui when the electronically controlled pump is activated
cheers

joojar · Jan 23, 2019

Friend who runs a garage once told me it's not uncommon for this to happen after bleeding the brakes *if* you used the full pedal stroke when you bleed.

Reason: rubber from the seals will gradually deposit inside the master cylinder with use, and will scrape to the extents of normal use over time. Eventually a ring forms at that point (e.g. at the point corresponding to half of full brake pedal travel).

If you then bleed the brakes with a full stroke, you immediately bugger up the master cylinder seals as they're forced over said ring of deposited rubber.

His suggestion to me: always bleed with a partial stroke, the process should still work and will give at least a shot at not mashing up your seals.

Alternative view: if your seals are that far gone, should prob be changed anyway.

discool · Jan 23, 2019

joojar said:
Friend who runs a garage once told me it's not uncommon for this to happen after bleeding the brakes *if* you used the full pedal stroke when you bleed.

Reason: rubber from the seals will gradually deposit inside the master cylinder with use, and will scrape to the extents of normal use over time. Eventually a ring forms at that point (e.g. at the point corresponding to half of full brake pedal travel).

If you then bleed the brakes with a full stroke, you immediately bugger up the master cylinder seals as they're forced over said ring of deposited rubber.

His suggestion to me: always bleed with a partial stroke, the process should still work and will give at least a shot at not mashing up your seals.

Alternative view: if your seals are that far gone, should prob be changed anyway.

It’s true, and well known to some of us.

It happened to a car I own when but it was 35 years old at the time, so I wasn’t that surprised, the car is now 41 so last year I used a pressure brake bleed kit for the first time.

With the disco when bleeding the brakes I just press the pedal halfway with my one person kit (a piece of tube with a ball bearing at the end) although that method means the job takes longer as the pedal is pressed twice as many times.

Sent from my iPad on a train

sierrafery · Jan 23, 2019

joojar said:
... then bleed the brakes with a full stroke, you immediately bugger up the master cylinder seals as they're forced over said ring of deposited rubber....

+1... same happens to the clutch cylinders too but it's about an edge of internal wear along the seal which ruins the seal

martin c brown · Jan 23, 2019

Thanks again thats got my thoughts going?,will look in to it and keep you up dated

Brakes

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