12 min read

W123 Mercedes Front Brake Replacement

Here is how to change the brake discs, pads and calipers on a W123 Mercedes, highlighting some of the things that can go wrong and what to do about it when they inevitably do...
W123 Mercedes Front Brake Replacement

The brakes on Nic’s W123 Merc hadn’t exactly been confidence inspiring, even when we first bought the car. I mean they worked, but there was the odd moment where they didn’t seem to be doing a lot or else they’d lock up momentarily under heavy braking.

Then when they started making a scraping noise it was time to have them stripped down and see what was going on. Well, to be honest it was long overdue but it was always raining or snowing, or she was out in the car so it kept getting put off. I know, it sounds silly to take chances but that’s how it goes in real life – if it’s not your car then you forget about it until something crops up to remind you. In this case it was Nic complaining about pedestrians stopping and staring at the yellow car making the funny noises as it slowed down. No more procrastination, something must be done!

Nic ordered a set of pads and I suggested some discs as well because the scraping sound told me that it might be too late to save them. Hopefully it should be obvious that you should always do both sides at once, that way you might have half a chance of stopping in a straight line.

I found a great guide to replacing the discs (rotors to any American readers) and pads over on the Peach Parts Wiki – thanks 79Mercy for the info. As it turned out, I ended up going further than the article so I will list the steps in full here in case anyone wants to follow along – do check the link as it has loads of useful pictures! I also learned a fair bit from watching Eric The Car Guy on YouTube – check him out, he’s much less annoying than most as he doesn’t keep announcing what website he’s from every five seconds.

So, first things first – wheel off. Remove the hubcap if you have one, and loosen the wheelnuts before you jack the car up. It’s a good idea to check your tyre thoroughly while you have it off the car, as there can be damage on the inner edge that might not be obvious otherwise. Like this:

A damaged tyre

Add one new tyre to the repair bill. Sigh. It’s a pain but better safe than sorry. Usually you’d replace the tyres in pairs if possible, but the other side had recently been replaced after a puncture anyway so we were all good there, and we made sure to get a matching tyre to replace this one.

Once the car is up on the jack, make sure to put axle stands underneath to support things – never work under the car using just the jack – especially if yours is as cheap and crap as mine!

Remove the sensors with pliers. On the Peach Parts article it says ‘9 times out of 10 they’ll break, don’t worry you have new ones’. This is probably true if you get Mercedes parts, but if you order from some online box shifter like we did you might find that you do NOT have any new sensors. We’ll worry about that later.

Depending on the type of calipers you have, there will either be some split pins holding the main pad pins in, or else they’ll just be held in by a metal collar at the back. If they have pins, remove them. Don’t worry if you break them, they should really be replaced anyway. You did buy some new ones didn’t you? Me neither. Doh.

Then with either type you should be able to tap them through the caliper with a punch or an old screwdriver. You should now be able to remove the spring that stops the pads from rattling. Finally, pull out the pads – if this is tricky, you may need to get some pliers/grips onto them and wiggle them back and forth.

So, this is how far you’d need to go for a standard pad change – this is what I found on our car.

The brake pads had definitely reached retirement age:

The calipers had well and truly rusted into their bores and the rubber seals are shot:

And the disc (or rotor as American types call it) had turned itself into some kind of heavy metal record, that I suspect if played backwards would tell me to check my mother-loving brakes more often:

All of this is on a car which has generally been well looked after and had regular servicing. Yikes!

Clearly the disc was going to have to come off, so I continued to follow the Peach Parts guide. To get it off, you first need to remove the caliper. This is held on by two 19mm bolts on the back – you can get to them with a breaker bar (which you will almost certainly need as they’ll be as rusty as the calipers are) – a good idea is to soak them with Plusgas at least 15 minutes before you try and remove them, preferably longer. A smart person would just reach around and soak them the night before without even removing the wheel. I’m not that smart.

Again, make sure the car is properly supported before you go ragging at tight bolts with a great big bar. Or don’t, and let Darwinism decide which of us was right. Up to you!

One important thing to note is that there are other 19mm bolts behind there – specifically one which is right next to one of the caliper bolts and forms part of the steering. It is very easy to get the socket onto the wrong one while you’re fumbling about behind the disc like a teenage lad trying to unhook a bra for the very first time.

If you look at the following picture, you should be able to see where the three bolts are (don’t worry, this is one I prepared earlier so yours won’t be like this yet!) – hopefully this will help get your bearings. The two shiny bolt holes are where the caliper was, and just below the bottom one is the other bolt. You really don’t want to undo this one – we did, by accident. Nic also broke it in two with her super strength. They are £15 + VAT each from Mercedes and the part number is A1269900801 (43mm). Think on.

We managed to get three of the bolts out, but the remaining one snapped off and one of the ones that came out ended up somewhat deformed. Replacements are available from Mercedes but you need to order them in packs of 5 for some strange reason. The part number is A1234210271 and they are £2.15 each at the time of writing, so a pack costs £11.64 including VAT. They’ll probably need to order them in which takes two working days. They come with thread lock already applied, which explains why the other ones are such a bugger to get off! Still, rather that than them falling out on the motorway.

So, at this point we had freed off the caliper so I suspended it with some bungee cords to stop it pulling on the brake hose.

This left us with one thread still embedded in the hole, so I drenched it in Plusgas again and went for a brew. When I returned, I got the Dremel (acually, a cheap crappy Silverline copy!) and a small grinding disc attachment and I made a groove in the sticking out part of the bolt, which I used to remove the remaining part with a big screwdriver. Points awarded.

Back to the Peach Parts guide then – remove the dustcap from the end of the hub with a hammer and a flat screwdriver or similar.

You might need a rag or two now to clear some of the grease – you should see a collar with screw which you can loosen with an allen key/hex bit – 5mm should do it. Don’t remove it, just loosen it. You should then be able to unscrew the entire collar piece from the hub.

The bearing comes off next – if you’re keeping it, make sure you don’t drop it onto the dirty ground or otherwise get crud in it. This would obviously be a good time to fit new bearings if you need them. I’ve spent enough so they can wait.

The disc should then pull away from the rest of the spindle, might take a bit of wobbling about to free it off but it’ll come.

Next we need to remove the bolts that join the brake disc to the hub, which require a 10mm hex socket. These are on TIGHT so you’ll be needing a breaker bar too. Again, Plusgas is your friend. The tip on Peach Parts was great – reattach the hub and disc assembly to the wheel with a couple of bolts, then you can get a grip on it much more easily. I did that, plus I propped it against a wall to stop it rolling so I could get decent leverage:

Once those are out, unbolt the wheel again and you should be able to separate the hub and disc – might need a few taps with a mallet or a rubber hammer.

If you’re just doing your discs and pads, then ‘reassembly is the reverse of removal’ as the books always say. However, in this case our calipers are shot so those need to come off too.

The first thing to remove is the 10mm bolt holding on the sensor-thingy – it’s a bit awkward so I had to use a spanner rather than a socket, wasn’t too tight though:

Next, clamp off the brake hose – you can buy clamps to do this, but I just used a rubber glove to protect the hose and a pair of mole grips:

Now you can remove the hose from the top of the caliper – you’ll need a spanner to crack this off (14mm I think) but once it’s loose you have to spin the entire caliper to unscrew it while holding the hose in place. You can see where it is at the top of the following picture (and also where the bleed nipple is in the top left):

If you’ve clamped up the hose properly and everything else is OK, you should now have a caliper in one hand and you shouldn’t be sitting in a puddle of brake fluid. Hurrah!

Next, I got on to www.brakeparts.co.uk and ordered two new calipers. They didn’t have exactly the same ones, but I spoke to Jason who was very helpful and assured me that the other type was interchangable. For reference, the ones on the car were BCA0632 and BCA0633 (Bendix/Bosch 18mm pads) and the ones offered were ATE BCA0756 and BCA0757. The ATE calipers come with a fitting kit (new pins and anti-rattle spring) but those sensors I mentioned earlier are BPW0115 and you need four. The calipers are on an exchange basis so you’ll pay a surcharge which will be refunded when you send your old ones back.

Right, so once the parts had been delivered it was time to get everything back together. First step, attach the hub to the new brake disc and semi-tighten the bolts. Reattach to the wheel like before and torque them up – the Peach Parts article recommends 84ft/pounds of torque. Finger tight won’t cut it. Lean the tyre against a wall like before to get good leverage.

Unbolt the hub assembly from the wheel again, and grease the spindle. Put the hub onto the spindle, then pack the bearing with grease and pop back into place. Screw the collar back onto the spindle but don’t to it too tight or you’ll crush the bearing. You want to get it tight and then back it off a little, about 1/3 of a turn. That should allow the bearing to spin freely. Tighten up the hex bolt that holds it in place, pack the dust cap with grease, and put it back on. Congratulations, you’ve changed your front brake disc!

Caliper next then – remove the transit bolt from the brake hose socket using a hex bit. Offer up the caliper to the hose and carefully rotate it to get it screwed on as tight as you can – don’t worry, you can nip it up later but you don’t want to put too much twist onto the hose so get it as tight as you can now. Reattach it to the hub assembly using new bolts if possible, blue locktite if not. These need torquing up to 84ft/pounds as well, at least according to the Peach Parts article and who am I to argue? Nip up that hose once the caliper is firmly attached, and remove the mole grips.

You can now test fit the brake pads to make sure they’re alright before you apply any copper grease. On one side of the car, this was fine. On the other, I couldn’t get the pads to go in for love nor money. In the end, I broke out the tape measure and here’s what I found:

Yup, one of the calipers has approximately 5mm less space than the other! They looked identical when I unpacked them but checking the box revealed that they’re slightly different. I got back in touch with Jason at www.brakeparts.co.uk and he kindly shipped me out a replacement straight away – top service, highly recommended! Once that arrived I removed the wrong-un and replaced it with the new one – success, the pads now trial-fitted fine. Here’s the different between new and old:

I put a little copper grease onto the pads where they might rub against the caliper (not on the brake material, obviously…) and put them into place. Next, the spring and pins – these needed gently banging in as they have a metal collar that holds them in place. Seems I didn’t need those split pins after all. Bonus! The spring is a little fiddly but basically it goes in BEFORE the pins and they tension it. Attach the sensors and that part is done:

If you’ve been paying attention, you should be looking at something a little like this:

But wait, there’s more! Having fitted new calipers, we now need to bleed the brakes. To do this, you’ll need a bit of tubing and a receptacle, or a thingy-bob for brake bleeding like I have – it’s basically a tube with a one way valve on it, you can pick them up at any spares place. It’s easiest to do this with two people – one to press the pedal, one to bleed.

Take the cap off the fluid reservoir and make sure there is plenty in – you do not want to run this dry! Loosen off the bleed nipple on one of the calipers, attach the bleeding tube and put it into a container – if you’re using a one way valve like me, it can be an empty bottle. If you’re just using a piece of tube, you’ll want to put some fluid into the bottom and submerge the tube so it can’t draw in any air.

Get your assistant to press the brake pedal while you open the valve, you should see fluid being forced down the tube. This fluid is almost certainly full of air bubbles. Keep pressing the pedal until the bubbles have gone and then close the valve – remember not to let the reservoir run dry. Repeat on the other side and you’re done! Don’t forget to replace the cap on the bleed nipple too, makes it much easier to bleed next time.

Once everything has been tightened up, wipe up any brake fluid residue and have a good few presses on the brake pedal to make sure there are no leaks from anywhere.

So next, wheels back on and start the car – check the feel of the brake pedal and make sure you’re totally happy with it before taking a gentle test drive. If anything feels amiss then don’t take a risk, check your work. It’d be a good idea to make sure nothing has come loose after a week or two of driving as well, better safe than sorry. Check for any leaking fluid at the same time.

After all this work,  the car is totally transformed. The pedal has great feel, starts working at the top of the travel, and the car comes to a halt in a nice straight line. Plus I feel much happier knowing Nic is driving a safer car. Definitely worth the effort!

Hopefully this has been useful to you, please let me know how your brake change worked out!