Lazy rear brake calliper?

[TomB]

Over winter I’ve taken the rear brakes apart to try and fix what looked like a lazy calliper. The inside of the left brake disc had evidence of only partial contact of the brake pad to the disc, as the shiny part of the disc was narrower than the right side. The rest of the inside of the disc was corroded, where the pad wasn’t sweeping the surface. 

I took the callipers off, and on the bench I took out the pistons to check for corrosion, cleaned the sliders and reassembled them with special grease for the pin sliders, and piston /pad interface. 

However, following reassembly with new discs, the witness marks on the new disc after a quick shake down,  again suggest exactly the same problem is still occurring. What should I be looking at next? New pads? Refurbished callipers? Or do they look ok and I should just keep an eye on them?

Old disc inside edge

New disc witness mark with pads, showing similar pattern with reduced contact of pads possibly apparent.

Other side of new disc for comparison.

[Ivaan]

Looks like the wrong size of pads to start with. Do they sweep the full width of the disc? What pads are you using?

[TomB]

Standard rears for Sierra as far as I know.  I've ordered a new set of pads from Redline, and Ill try bleeding again.  Something isn't right.   

[ScottR400D]

So was that with the old pads? I would have thought new pads were the first step. 

[Youds7]

You should always fit new pads when replacing discs

[TomB]

School boy error I guess. The old pads had plenty of life left and they’re flat against a steel ruler edge, so thought I could reuse them. I assumed a sticky calliper, so a clean up would help diagnose. Guess it now pads or bleeding issue. Someone on the FB technical group suggested the piston seal plays a part in pulling the piston back into the bore when th brakes were released. Not sure if that could be something? If pads and bleeding again don’t get better wear/ contact, I guess I’m going to try a new / rebuilt calliper. 

[SM25T]

Were the pips on the rear of the pads engaged in the slots on the piston ... ie at 12-3-6-9 o'clock

[TomB]

I think so, but I’ll be investigating shortly. However now I’m worrying that I’ve got the wrong pads coming!

My fronts brake are HiSpec 4 pots, running on standard discs with the larger Elise pad in Mintex 1144 compound, with standard pads on the rear. 

When I spoke to Redline today, and explained what my front brakes were, Chris thought Mintex 1144 on the rear would be good. I lock up my fronts in a hard brake/ emergency, which he thought meant the rears weren’t working hard enough. But fitting Mintex 1144 on the rear doesn’t appear as a recommendation inn these pages, and it’s contrary to most opinion with Mintex in the front and standard on the rear. However with my slightly unusual front brake, I not sure if fitting these on the back is the right choice.

Any thoughts?

[Beagler]

Seems there is something other than pads scoring the discs. There are two distinct lines on the new disc.

[TomB]

Well hopefully the new pads will fix it 

[Englishmaninwales]

Tom I think SM25T is on the money, pip is not engaged in the piston slot and inner pad is tilted.

[TomB]

I've fitted the new pads over lunchtime (still WFH mostly  ).  

The old pads were both located correctly; however, the inner LHS is noticeably not flat against a steel ruler.  So that pad was not helping.  The new ones comfortably cover the witness mark on the new discs, so hopefully that's a lesson learnt - don't be a tight arse when changing discs, even if the pads have plenty of meat left.  Just hope the Mintex 1144 work on the back.   

I might just check the bleeding again tonight before I put the wheels back on.  MOT tomorrow so can't do more now.