Sinking break pedal after disc change

**Chappers** · 07-02-2011,07:17 PM

I put new front discs and pads on and eveything was fine. A few days later i changed the rears and now the pedal is sinking but gets hard when you pump. No loss of fluid, i didnt undo any pipes, just unscrewed the resovoir cap and wound the pistons in to fit new pads, any ideas

many thanks, Chappers

**Peter D** · 07-02-2011,07:51 PM

Bad move in my book. When pressing/winding pistons back, you should fit a bleed pipe and crack the bleed nipple open, press the piston back slowly, close the nipple, fit the new pads and rebuild then bleed that calliper in the normal way and finally lock the bleed nipple off. You have forced old contaminated fluid back up the system.
Make abolutely sure you are not leaking fluid at the rear banjo union. Common problem..
To recover this suck out all the fluid from the brake resorvior and fill with fresh fluid then bleed all four callipers. Use an assistant and do not use the brake travel past 2/3rd travel. I assume, as a side issue, that you did put the front inner pads in on the correct side. They are sided and marked with an arrow of direction of rotation of that wheel. Get it wrong and the pads can knock every time you apply the brakes. Regards Peter

**Chappers** · 07-02-2011,08:18 PM

Pete,
thanks for the reply, i will try that, the fronts seem ok and i dont recall any directional arrows on the pads but i will check. I thought it might have been the master cylinder but hope its as you suggest

cheers Paul

**Peter D** · 07-02-2011,08:26 PM

What make where the pads, I have seen after market pads with no marking but many with as are the OEM ATE pads. Some pad sets come with instructions that detail the direction arrow and it's location.
As you did not open the bleed nipples or pump the peddle why would the MC fail. I would be more concerned they you may have forced debris backwards through the ABS and if you knew what that cost you would open the bleed nipples. Regards Peter

**bigtimz** · 08-02-2011,07:50 AM

This sounds the same as i recently had on my old M Benz 190.
The brake master cylinder was the cause.

**iwaters** · 08-02-2011,10:41 AM

Originally Posted by Peter D

What make where the pads, I have seen after market pads with no marking but many with as are the OEM ATE pads. Some pad sets come with instructions that detail the direction arrow and it's location.
As you did not open the bleed nipples or pump the peddle why would the MC fail. I would be more concerned they you may have forced debris backwards through the ABS and if you knew what that cost you would open the bleed nipples. Regards Peter

Thanks for this Peter. Elsawin doesn't mention to open the bleed nipples but at least I know now for future.

Cheers

**Peter D** · 08-02-2011,11:28 AM

Elsawin does not tell you to tighten a nut clockwise either, but there are some things you just have to know. Not only does undoing the nipple allow you to expell the contaminated fluid but it breaks the corrosion that can cause a nipple to sieze and then when you need to bleed the system you snap one off. Undo, clean, a little copper ease and do up when bled. Regards Peter

**zollaf** · 08-02-2011,11:31 AM

very worthwhile advice there.

**iwaters** · 08-02-2011,11:33 AM

Originally Posted by Peter D

Elsawin does not tell you to tighten a nut clockwise either, but there are some things you just have to know. Not only does undoing the nipple allow you to expell the contaminated fluid but it breaks the corrosion that can cause a nipple to sieze and then when you need to bleed the system you snap one off. Undo, clean, a little copper ease and do up when bled. Regards Peter

True. But there are pages on how to remove a wheel though which you'd think even the thickest fitter at the main dealer could manage.