Quick answer please

[rab60bit]

Doing a rear brake pad change on my Contemporary SE and first tried to remove the guide pins since this is the quickest method. I've a wide range of allen keys but I'm stumped with what size fits the pins - it's smaller than 8mm and none of my imperial selection seem to fit - what the blazes size is it?
I've now got the rear O/S free by removing the two calliper mounting screws but would like to dismantle the guide pins.
Both O/S pads are equally about 1/2 worn but I'm changing the rears because a) the December MOT advised "rear pads low"
b) the N/S started to make a scraping noise
I'll soon know for sure but I'm expecting the N/S pad(s) to be unequally worn, is there a common reason for this?

[R75800sd1]

The pins are 7mm hex like many BMWs. The calipers wear eventually and the pistons stick, although in my experience I wouldn't say it was common. Spray some WD40 under the rubber before you try to push the piston back.

[SD1too]

Originally Posted by rab60bit

... it's smaller than 8mm and none of my imperial selection seem to fit - what the blazes size is it?

Quick reply from memory John; it's 7mm, a size which is seldom included in Allen key kits.

Originally Posted by rab60bit

I'll soon know for sure but I'm expecting the N/S pad(s) to be unequally worn, is there a common reason for this?

Different application methods John; the inboard pad is moved directly by hydraulic pressure via the piston whereas the outboard relies upon the sliding caliper.
The piston circumference can become dirty or rusty thus increasing friction and the sliding surfaces of the caliper do corrode as they're unfinished cast iron.
A small difference in pad wear should not be a cause for undue concern.

Simon

[rab60bit]

Originally Posted by SD1too

Quick reply from memory John; it's 7mm, a size which is seldom included in Allen key kits.

Different application methods John; the inboard pad is moved directly by hydraulic pressure via the piston whereas the outboard relies upon the sliding caliper.
The piston circumference can become dirty or rusty thus increasing friction and the sliding surfaces of the caliper do corrode as they're unfinished cast iron.
A small difference in pad wear should not be a cause for undue concern.

Simon

Well that was a disappointment. O/S rear pads were about 2/3rds equally worn but the N/S pretty much equally worn and down to the metal (hence the reported scraping noise!) - reason, bellows half hanging off in the breeze(but intact) and piston partially rusted/siezed. All slider pins are shiny and smooth/no ridges etc. I've renewed the O/S pads, cleaned-up the N/S piston as best I could and moved the old pads to the N/S pro tem. and everything seems OK. I now need a caliper service kit, who are the best and what level of kit should I go for?
Do we clamp-off the brake lines (mine seem a bit too solid to compress..?) or just drain down/release pressure from the bleed nipple and then do bake fluid recharge...?

[SD1too]

Originally Posted by rab60bit

Do we clamp-off the brake lines (mine seem a bit too solid to compress..?) or just drain down/release pressure from the bleed nipple and then do bake fluid recharge...?

You're absolutely right John, the rubber brake hose is very unyielding and so I'm never very happy about squashing it since (1) it's rarely 100% successful in stopping flow and (2) there's a risk of permanent deformation.

You can use the Haynes recommendation of unscrewing the master cylinder cap and placing a sheet of plastic over the top of the filler neck then refitting the cap. This will stop the application of atmospheric pressure to the fluid in the reservoir with the result that it stays put! I find that this method works really well.

When your repair work is complete you will, of course, be bleeding the rear brakes anyway.

Thanks for posting the update. I wonder how the piston seal became damaged?

Simon