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Rear brake trauma !

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Old May 13th, 2019, 12:39   #1
JGEM
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Location: Christchurch NZ
Default Rear brake trauma !

OK, so I've changed the rear discs and pads on my XC60 twice before over the last 5 years so this was to be the 3rd time tonight and I was confident that I knew what I was doing, which has made this tale of woe below somewhat embarrassing and annoying !

I used my Vida DICE unit to put the parking brake into service mode (I heard the motors whir and then removed the old pads. I was going to also change the discs but upon inspection they were not scored and within thickness spec so I decided to just replace the pads and save time. (First time I have ever done brake pads and not changed the disc at the same time) . Before I fitted the new pads I used a tool to depress the piston down until it stopped.

The tool was just a simple one like this and has always worked fine and having just a small turn handle means that you cant apply excessive force

https://www.amazon.com/Autotools-Spr.../dp/B007N3I4QY


OK, so the piston was bottomed out as much as the tool would comfortably take it down and I realised that the new pads plus disc would not fit in the caliper opening width available.

It was like service mode had not adequately retracted the caliper piston.

This is where it all went pear shaped....

I went back into Vida and it confirmed that parking brake was in service mode. Then I made my BIG MISTAKE.

Without thinking clearly I thought that maybe I should exit service mode and then reenter service mode and then second time around the piston would maybe magically retract more. Stupidly i did this sequence of events with the caliper with no pads in it and not on the disc. When it exited service mode the piston wound out and out and out until POP - it came completely out of the caliper and a small gush of hydraulic fluid went onto the garage floor. Disaster ! (Very similar to cases I have heard of with other cars where someone depresses the brake pedal with the caliper removed from the disc and out pops the piston.

I have a relatively clean work environment so my initial thought was to re-insert the brake piston and bleed the brakes. This is where the puzzling part happens - the piston will not go back into the caliper. I thought initially it was due to the threaded insert that goes inside the brake piston (which i had wound fully back) so as a trial I removed that threaded square "nut" bit that goes inside the piston altogether and tried pushing in the piston into the caliper by hand but still no luck - it just wont go in. I suspect I have the piston misaligned but even with careful pushing and wiggling I just cannot get the darn piston to go down. Maybe it is air trapped behind the piston and I need to bleed this out first ? Will try again tomorrow night


Anyway, brakes being serious stuff I have somewhat lost confidence with the rear caliper as my misadventure could easily have introduced contamination into the system. It has certainly introduced lots of air !

A good Youtube tutorial on bleeding brakes here

https://www.youtube.com/watch?v=cpKQVq9CEZE

My plan now is to get the caliper off the car and clean and inspect and reassemble it on the work bench. If I have any reservations about it then I will replace it with a new one .

The only silver lining to this mishap is that I now have an excuse to buy a power bleeder !
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2014 XC60 D5 AWD
Previous: 1994 940GL wagon, 2008 S80 3.2 AWD, 2004 V50 2.4, 2009 XC60 D5 AWD
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Old May 14th, 2019, 10:11   #2
jamie1131
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Default

try removing the parking brake motor from the back of the caliper, you'll see a spline bolt which the motor drives. See if you can back that all the way off or if it maybe came to the end of its travel and got stuck when you exited service mode.

no idea if this will work but its worth a try!
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Old May 17th, 2019, 19:53   #3
Leodis
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If you haven’t fixed this yet, open the the bleed nipple first as without doing so you are trying to overcome the friction of the seals and compress the trapped air.

Good luck.
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Old May 19th, 2019, 22:04   #4
JGEM
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Default Back on the road again

I gave up for the moment as even with bleed nipple open and with the motor removed and parking brake wound back as much as possible the piston was still jammed. I just swapped the caliper for a good used one from a car wrecker, bled all 4 wheels and I'm back on the road again. Will investigate at a later stage as to what ails the old caliper.
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2014 XC60 D5 AWD
Previous: 1994 940GL wagon, 2008 S80 3.2 AWD, 2004 V50 2.4, 2009 XC60 D5 AWD
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