As the title suggests, I'm going to replace the discs and pads all round on my W639. Are there any 'gotchas' such as special tools etc that might make me get it done at a garage or is it straight forward?
I don't think there are any special tools required as such. However whilst doing the brakes is a straight forward enough job if you are asking for advice at this stage I would seriously consider getting a friend who knows how to do them to help you because if you don't put them back together properly, don't torque the caliper/carrier bolts up properly, miss a pad retaining pin/spring you could find yourself with brakes that fail when you need them. By all means have a go yourself as its the best way to learn but please if you are unsure, get someone who knows to help you.
Things to watch out for, on my 108cdi one of the rear caliper bolts was seized solid and snapped and I had to drill it out and helicoil it. So if it feels stupidly stupidly tight it's worth using lots of good penetrating oil and make sure you have time to spare.
Replaced the rear discs and pads with no problems. Only thing now is that the parking brake has gone a bit pants - even though I didn't even touch the shoes etc etc (just slipped the new disc on). Is the parking brake self adjusting or will I need to adjust it manually?
You will need to do it manually. A blade screwdriver through one of the wheel bolt holes does the job. Adjust them so they are *just* binding on the disc with the handbrake off then back them off a touch to get the correct travel / feel at the handbrake lever. It takes a few days or so for the new handbrake shoes to bed in with the new discs properly.
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