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[DanielCoffey]

I am looking at the section in the Unofficial Manual concerning Front Pad replacement on a 2018 DS and I have a question about one of the steps...

LINK : https://zeromanual.com/wiki/Front_Brake_Pad_Replacement

Step 7 : Fully push back the pistons, by using a C-clamp with at least 2x2-inch opening and 1-inch heads, clamping the old pads to the outside caliper body.

What is the purpose of this part? Is it because the pistons have a tendency to push forwards as soon as the caliper is off the rotor? What is the tool being described as I don't understand what it is? Could a slim wedge of roughly the same thickness as the rotor do the same job?

[Doug S]

As your pads wear, they get thinner, so the pistons have to extend outward to keep the pads pressed to the disk. You'll notice the fluid in the reservoir getting lower as that happens.

When you install new pads, you need to force the pistons back into the caliper, back to their "new-pad" position. This involves pushing fluid back through the hydraulic line into the reservoir, and requires some force to do. You can usually do it just with your hands, but a clamp makes the process more controlled and gradual.

That's also why you should remove the cap to the reservoir before installing new pads. That keeps an "air spring" from forming in the top of the reservoir and making the piston retraction process more difficult.

Edit: Also, DO NOT put ANY pressure on the rotor as you work on the brakes! You'll knock them out of true, and even a couple of mils will make  your brakes pulse or worse. Keep those babies as straight and true as you possibly can; they're plenty good at going out of true all by themselves.

[DanielCoffey]

Thank you - that is a good explanation. I will give it a go once the pads arrive.

It is a shame that the front pads can't be slipped out of the caliper in the same way the rear ones do. That was a really easy swap.

[Richard230]

Thank you - that is a good explanation. I will give it a go once the pads arrive.

It is a shame that the front pads can't be slipped out of the caliper in the same way the rear ones do. That was a really easy swap.

The front pads can be removed and reinstalled without removing the caliper.  Just remove the retaining pin at the bottom rear of the caliper after removing the small C-clip and they practically drop out.  I am sure that there are more detailed instructions in Brian's unofficial on-line maintenance manual.  But if you are installing new pads you will have to push the caliper pistons back a little to get the new pads installed. I find that I can push the pistons back with my fingers after loosening the master cylinder cap.

[DanielCoffey]

Ah, I understand.

So the actual reason the Unofficial Manual suggests taking the calipers off is for the situation where the old pads are well worn and the pistons NEED to be pushed back. If the pads are only slightly worn and are merely being upgraded or replaced due to damage then you might be able to get away with undoing the bolt, dropping the old pads and gently pushing back the slightly extended piston by use of a fingertip or perhaps the end of a plastic ruler.

Well my new pads will arrive next week so I'll report on how I get on.

[BrianTRice@gmail.com]

I'll see whether the procedure in the unofficial manual can be clarified based on this feedback, thanks.

[Doug S]

I always remove the caliper anyhow. It's just a couple of bolts, and it ensures any pressure I apply doesn't wind up warping the rotor. You might think that thing's a very thick piece of steel and would be hard to deform, but you'd be surprised. Ask me how I know.

It's also a good opportunity to make sure the caliper slide moves freely, which is also important to ensure no sideways pressure winds up being exerted on the disk when braking.

[Doug S]

Perhaps it should also be pointed out that this is why you should top off the reservoir AFTER installing the new pads, and applying a few braking cycles. If you top off before, then install the new pads, retracting the pistons can easily overflow the reservoir. Replace the pads first, button things up, do a quick test ride with a couple of braking cycles to get everything seated, then top off the reservoir to the top line.

[Doug S]

And while I'm on a roll, one final tip: Don't let the caliper hang by the hydraulic cable. It's pretty heavy, and there's no reason to encourage the hose to develop a leak. Get an upside-down bucket or something to rest it on while you're not holding it.

[Richard230]

And while I'm on a roll, one final tip: Don't let the caliper hang by the hydraulic cable. It's pretty heavy, and there's no reason to encourage the hose to develop a leak. Get an upside-down bucket or something to rest it on while you're not holding it.

I hang the caliper using a bungee cord attached to the turn signal stalk.