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Front calipers not releasing - servo piston not coming off??

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colmcd

Member Since: 04 Jun 2021
Location: north lanarkshire
Posts: 9

2006 Discovery 3 TDV6 HSE Auto Java Black

Front calipers not releasing - servo piston not coming off??

Hello again. My brake woes continue. I do not have a LR dealer or specialist close by so rely on local lads for work on car but they do not want to touch this.

Having changed master cylinder etc the front calipers now clamp on and wont release when driving. Wheels getting seriously hot etc. Local lad has had a good look and advises the calipers are working normally when engine is off, the wheels turn ok and the pistons move out and in properly. Problem comes once engine on and servo operating. Pistons come on and remain on as fluid pressure not being released when brake pedal released.

He thinks that the servo piston that goes into the master cylinder is pushing into the cylinder and not releasing so fluid never flows back. Before I remove the master cylinder again and have to bleed brakes yet again can anyone confirm the diagnosis and give a fix? Or has anyone any other ideas about what could be causing this situation?

Thanks in advance for any help given.

12th Jul 2021 3:10 pm

Pete K

Member Since: 15 Jan 2016
Location: GL
Posts: 10681

2005 Discovery 3 TDV6 HSE Auto Rimini Red

Well to me problem could be

a) servo or master cylinder
b) ABS or electric problem
c) caliper or hose problem

a you could try disconnecting the vacuum pipe and drive a short distance, slowly and gingerly and see if the problem is fixed. Brake performance will be significantly reduced and hard pedal.

b you could unplug the ABS unit and drive and see how it is.

12th Jul 2021 4:33 pm

lynalldiscovery

Member Since: 22 Dec 2009
Location: Maidstone
Posts: 7274

2006 Discovery 3 TDV6 Metropolis LE Auto Bonatti Grey

Master cylinder free play?

12th Jul 2021 7:48 pm

colmcd

Member Since: 04 Jun 2021
Location: north lanarkshire
Posts: 9

Thanks both. It is a new master cylinder so not sure about any free play. The ABS unit has previously been sent away for testing/repair. Returned as no faults found. Front calipers that are locked on are both brand new as are all the hoses. Garage says non of those are the problem as far as they can tell.

My son has also suggested the drive with vacuum disconnected. So may try that and see what happens. The car should move more easily from the outset, so feel alone should tell me quite quickly if the calipers are released. Otherwise need to drive for a while to see if wheels get hot or not. Not easy around these roads unfortunately.

Son lives some distance away but will be visiting in couple of weeks. When he is able to help am going to take master off again, compare to old one to see if travel etc of piston is different and check the way the arm in the servo moves. Depending on results will then reinstate and bleed again using pressure bleed rather than pure manual. Until then it will be shanks pony/public transport for me.

12th Jul 2021 9:15 pm

Pete K

Member Since: 15 Jan 2016
Location: GL
Posts: 10681

Are you aware when disconnecting the master cylinder you need an electronic diagnostic tool to bleed the abs pump ? Possibly even the issue ?

The expensive £500 gap iid will do it
Or maybe even the Autel ap200 at £48 direct.

https://www.autelstore.co.uk/wholesale/aut...anner.html

Last edited by Pete K on 13th Jul 2021 9:02 am. Edited 1 time in total

13th Jul 2021 7:28 am

jenseneverest

Member Since: 12 Jun 2017
Location: somewhere
Posts: 769

2005 Discovery 3 TDV6 Base 7 Seat Manual Zambezi Silver

the autel works, still need 2 of you one for pumping.......

13th Jul 2021 8:13 am

colmcd

Member Since: 04 Jun 2021
Location: north lanarkshire
Posts: 9

Thanks. I had read about this but others had said they had managed to do the bleed successfully without the tool. Maybe I am one of the unlucky ones. Do not do enough work on car to warrant the cost of the GAP but if the Autel works I can run to its cost.

Son is bringing his pressure bleeder that works from air pressure from tyre. Can you use that along with the Autel?

Going to try a short drive this morning with servo disconnected to see what happens.

13th Jul 2021 8:55 am

Pete K

Member Since: 15 Jan 2016
Location: GL
Posts: 10681

https://www.autelstore.co.uk/wholesale/aut...anner.html

As long as you aren't touching the ABS or master cylinder, you don't need the electronic tool. But you have.

Yes you can use a pressure bleader with it.

I recently did a brake job, and found using one of them cheap vacuum suckers off ebay at the caliper was quick and easy for removing air. But that is another issue

13th Jul 2021 9:06 am

jenseneverest

Member Since: 12 Jun 2017
Location: somewhere
Posts: 769

i blew the clutch master cylinder feed off the reservoir with one of those..... so be careful with the pressure type bleeders !! ( or cable tie the stupid plastic pipe like i did on mine)

EDIT
To bleed correctly with the diagnostics, you need to pump the pedal and do each wheel in order.

13th Jul 2021 9:13 am

Pete K

Member Since: 15 Jan 2016
Location: GL
Posts: 10681

Apparently the tyre pressure should be maximum of 20 psi

13th Jul 2021 9:29 am

jenseneverest

Member Since: 12 Jun 2017
Location: somewhere
Posts: 769

yup i had, guess it would not even be an issue on an auto.

13th Jul 2021 9:46 am

colmcd

Member Since: 04 Jun 2021
Location: north lanarkshire
Posts: 9

All very helpful information. I have ordered my Autel unit and will see where we get to in a couple of weeks time.

13th Jul 2021 10:15 am

Globetrotter448

Member Since: 21 Mar 2017
Location: Londonderry NSW
Posts: 1818

2007 Discovery 3 4.0 V6 Petrol SE Auto Java Black

Have you checked to see if there is any oil in the servo. My guess would be the ABS unit not being bled fully.

18th Jul 2021 8:56 am

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