Restoring rear end balance
Nigel treats his 325i to some new brake pads
At a recent MOT, the tester advised that my workhorse BMW 325i had a couple of problems with the rear brakes. On the test rollers, the handbrake showed an imbalance, with the left side producing less effort than the right. The disc pads were also worn, not dangerously so, but still needing replacement before long. Neither was a failure point, but both drew advisory notes on the pass certificate.
Not wanting to run around with a less than perfect braking system, I ordered a new set of rear brake pads online plus a pair of retaining clips. The brake discs themselves showed only slight wear and could be reused. The BMW’S handbrake operates via a small set of shoes inside a drum which is cast into the back of the brake disc. I had replaced the handbrake shoes around a year ago and adjusted them carefully, or so I thought at the time. Clearly all was not well; a likely explanation is that one of the rear disc calipers had been binding slightly, which would have confused the handbrake adjustment. A binding caliper would also have contributed to rapid wear of the disc pads, so there could be a picture emerging of why there were two simultaneous problems with the rear brakes on my Beemer.
Not difficult
Replacing the brake pads isn’t tricky, but the calipers need to be removed. After raising the back of the car and securing on axle stands, I released the pads retaining clips and undid the two guide pins that secure each caliper. All came apart without a struggle, as I’d stripped this out to change the handbrake shoes last year. Next, the calipers could be prised off their mounting brackets and away from the discs, then supported with wire to avoid placing strain on their hoses. The left side pads were badly worn and starting to fall apart, though the right side looked only slightly down, again suggesting the left side caliper may have been sticking. When I pressed the caliper pistons
back ready to fit the new pads, both sides retracted easily, without any stiffness.
I worked the left caliper piston back and forth a couple of times and all seemed well. If the caliper had actually been sticking, pressing its piston back seemed to have freed it up.
Meanwhile, with the calipers out of the way and pads removed, I could have a good look at the handbrake’s operation without the risk of binding brake pads confusing matters.
The handbrake shoes are adjusted by turning a small, knurled wheel inside the brake drum. Access is tricky; a screwdriver blade has to be poked through one of the roadwheel bolt holes to flick the adjuster round one click at a time. Following much fiddling, going back and forth readjusting each side, I was able to get a reasonably well balanced action from the handbrake, with the lever pulling up six or seven clicks to full engagement and the rear hubs locked up tight. Reassembly was straightforward. I cleaned the caliper guide pins thoroughly and gave them a smear of copper grease before refitting and torquing up to the workshop manual setting of 35 Nm. The last jobs were to fit new brake pad retaining clips and reattach the brake pad wear sensor wire.
Wheels refitted and their bolts torqued up, I lowered the car back to the ground and went for a short test drive. The brakes feel fine and stop the car quickly in a straight line, though they did that before. There’s no question the handbrake now feels more effective, but I’m still not absolutely convinced I’ve found why the brake pads wore down faster on one side. Next month I will need to swap from summer to winter tyres and while changing the wheels, I will take the opportunity to have another good look at the rear brakes and handbrake. I hope to avoid replacing that suspect left-hand brake caliper, but that could be the next step if the rear brakes are less than perfect.
‘Pressing the caliper's piston back seemed to have freed it up’