Brake caliper failures and how to prevent them

[FireFox31]

Brakes are critical to the safe operation of our cars. During my past 20 years of P80 ownership, the part with the highest failure rate I've experienced is rebuilt brake calipers. Let's use this thread to document the various ways a brake caliper can fail and how to proactively maintain them to prevent failure.


Future content:
I will eventually expand this post to include all the failures I've experienced - dust boots not seated correctly by the rebuilder, splits in the dust boots, wear on the dust boots, rust holding the pads in place, rusty pins and springs, kinked brake fluid hoses, etc.

I will also post my recommended proactive maintenance checks and ideal behavior for calipers - Remove pads, insert wood blocks, and press pistons out slightly with brake pedal or compressed air to inspect the boots. Don't press pads out too far or the boots will stretch. With bleeders open or calipers disconnected, pistons should be able to be pressed in with a strong hand, not requiring a pad spreader.


Easy diagnostics:
For my recent (10/2024) caliper failure, I noted a squeak while driving from its position on the car. Since I was too busy, I didn't look in to it until a stiff piston jamming the pad on the rotor destroyed them both. When you hear a squeak, check it out.

Check for stuck calipers pressing pads to rotors by rolling to stop very slowly in neutral on level ground. The car should roll to a gradual stop, even rolling back after stopping if the ground isn't totally level. If a pad is stuck to the caliper, the car will roll almost to a stop, then immediately stop because slight breaking force is being applied; and certainly won't roll back on near-level ground.

Another check for stuck calipers is, quickly after coming to a stop, to hop out and put your finger near the caliper and rotor. Excessive heat indicates a dragging brake pad and should be investigated. Don't touch anything because these parts can get extremely hot, especially if they're dragging.


Proactive protection:
I suspect that the best protection is to paint or powder coat the calipers. That thread documents how I got mine stripped and coated, but I haven't finished reassembling them and putting them in to service yet. I'll update it once I do.

My current recommended rebuilt caliper are NAPA Adaptive One (formerly NAPA Total Eclipse) which are Cardone premium rebuilt. They have a limited lifetime warranty through NAPA for as long as you own the car. (edit 10/10/24) "Limited" because they allow one free replacement, after which you must buy the next one. They are powder coated, though that eventually develops rust beneath it and flakes off. I examined one from a neglected car with three failed calipers. The NAPA had driven 3000 miles in 1.5 years, likely as the only working caliper, then sat with the car parked on a lawn from late Spring to early Fall. After that, I examined it and found there was no rust on the body, the boots were perfect, the pistons ejected easily, and they could press back into the body with my bare hands. By contrast, my newly rebuilt Centric caliper bodies showed surface rust in a week of no driving and one rear caliper piston seized extended after 15 months.


Dust boot failures:
A recent failure I experienced was a Centric rebuilt right rear caliper on a 2000 V70. The outer piston froze pressing the pad to the rotor, making the wheel almost impossible to move by hand while off the ground. The only symptom was a slight squeak when coming to a stop and a longer squeak when driving in reverse. Luckily, the failure was caught early.

I believe the failure was caused by water entering the cylinder after the thin dust boot rubber peeled off the internal metal sealing ring. The sealing ring was rusted so perhaps it expanding outward ripped the boot. There was significant rust on the caliper groove which holds the boot so water could have also entered there.

(edit 10/10/24) My most recent failure was a NAPA Adaptive One rear right caliper on a 2000 V70. Both dust boots had simply worked loose from their grip on the caliper body. The catch was they had internal metal rings, not the external snap rings I've received on other NAPA calipers. This one had a date code of 2019 on a sticker though I bought it in 4/2023. Otherwise, the caliper looked great, boots weren't cut, and pistons had only slight rust (I caught it early) and could be retracted with a pad spreader. But they wouldn't retract on their own, jamming the outer against the rotor, destroying it. So, consider replacing internal ring boots with snap ring boots.

To proactively monitor for failure, it helps to inspect the dust boots thoroughly. On the rears, it might even help to remove the boots and inspect the groove which holds them (they're easier to remove and replace than the fronts). Replace if the boots show any break, if the grooves have rust, or if the pistons can't be pressed back in with a strong hand with the bleeders open.

[FireFox31]

Reserved for boot splitting.

[FireFox31]

Examining a caliper piston:
You can determine if your caliper has failed, can be repaired, or is working fine by examining the pistons for rust. I learned about piston removal from cn90's excellent front caliper rebuild instructions. I plan to expand on his work here and add info for rear caliper disassembly (which is easier than the front).

I recommend rebuilding a caliper which has suffered minor damage because it will allow you to keep a caliper which is functioning well and has proven its quality in real world use. Your current caliper with minor damage may be better than a poorly rebuilt caliper that you buy these days. I have a set of rear calipers which look terrible but their pistons are spotless and move well so I simply replaced their boots (and may even sand blast and paint them for full restoration). Also, rebuilding will save you money of buying new calipers and shipping back the cores.


The basic procedure to eject rear pistons:
1. Press the brake pedal half way and hold it there by placing a board between the pedal and the front of the seat. This closes "the valve" preventing any more brake fluid from dripping out of any disconnected lines.

2. Connect a catch tube and container to the caliper bleed valve, open it, and let the small amount of fluid out due to pressure from the pedal being pressed.

3. Remove a caliper the car. If the fitting is stuck, don't remove it unless you have a new M10 x 1 bubble flare fitting and bubble flare kit on hand to replace the ruined fitting, and an M10 x 1 tap to bore out the caliper hole to accept a new fitting. See that post in this thread for fitting replacement.

4. Drain the fluid from the caliper. Use a pad spreader (or, ideally, your bare hands) to press the pistons in, forcing the fluid out.

5. Remove the boot from the caliper by carefully prying it up from the grove in the caliper body. Some boots have a metal ring around the outside so remove that first. Some boots have a flat metal ring inside the boot grabbing the caliper body, so be sure that you are prying under that. Leave the boot connected to the piston.

6. Place a C clamp on one piston to hold it in place. One moves easier than the other because it's closer to the fluid input line but I forget which that is off hand and will update this later. Put the clamp on the easier piston to force the harder one out. Put a block of wood between the free piston and the clamp so that the piston can only eject a fraction of an inch. Have a few successively thinner blocks of wood (or a set of shims which you can remove one) to gradually step the piston out.

7. Use shop air on a trigger with a cone attachment pressed into the fluid input hole. Cover the hole and air gun with a rag because vaporized brake fluid will shoot out. Wear eye, mouth, and hand protection and cover your work surface to prevent that brake fluid from getting everywhere. Give a gentle press on the trigger to force the piston out a fraction of an inch until it hits the wood block. It may slam out with some force! Remove the block and put in the next thinner one (or remove one of the few shims), allowing it to move out another fraction of an inch. Apply air to push it out again. Repeat until it's far enough out that you can remove it by hand.

8. Examine the piston. If there is any rust, scratches, pits, or other imperfections, the piston will need to be replaced. Examine the caliper bore. I've never seen a bore with problems, even one which had a rusty piston. I suspect that if the piston is bad, the bore might have sustained damage, thus the whole caliper must be replaced. I could be wrong about that.

9. Grease and reinstall that piston. It should press in by hand or with slight force from a pad spreader. Reattach the dust boot. Swap the C clamp to that piston and use the procedure above to eject the other piston and examine it as well.


Replacing a caliper boot:
When the pistons are clean, I recommend replacing only the worn dust boots with the ATE rear boot kit from FCP. See the cn90 post above for the tricky front boot installation procedure. Rear boots are easier: Eject the piston (See above), affix the boot to the piston, press the piston back in the caliper, then affix the boot to the caliper.

[FireFox31]

Reserved for piston rust.

[FireFox31]

Fittings seized:
On rear calipers, the brake line fitting can become seized inside the caliper. I suspect the hole for the fitting has shrunk due to expansion from rust, grabbing the fitting. Removing the fitting will flatten its threads so it won't fit in a new caliper. A new caliper will require a new fitting to be flared to the line.

Conversely, the old caliper fitting hole is contracted from rust so a new fitting won't fit in it. The caliper fitting hole must be tapped (likely M10 x 1) to expand it for the new fitting. Before trying to remove a rear caliper, have on hand M10 x 1 bubble flare fittings, flaring kit, and a magnifying glass to inspect your flare.

Flaring on a new fitting:
Resources to learn about flaring new brake line fittings:
BMW brake line replacement by cn90. The BMW uses similar lines and fittings so the relevant bits apply to Volvo.
Brake line flaring summary by polskamafia mjl
Brake line flaring by vjaneczko
Brake line flaring video

Brake line and fitting info:
Volvo brake lines are 4.75mm, 3/16" lines.
On my 2000 V70 NA FWD, the left rear brake line is 24 1/4" excluding fittings, right rear is 32 1/4" excluding fittings.
The "bolt" end fittings are M10 x 1.0 mm for 3/16" line with bubble flare with a flat tip in bright brass. These brass fittings are soft and easily destroyed
For fittings, my local auto parts store had in stock SURR brand BR255 M10x1 for Ford, brass colored, tall hex head, flat tip for $2 each. There are also SURR BR230 but their hex head is really small and the tip seems angled inward, and SURR BR250 which is blue and too long with the angled tip.
Another way to get fittings is to buy a short length of brake line already flared with two fittings on it. Cut off the fittings, then use that brake line to practice flaring.

Remember that a new fitting may not fit in a rusty old caliper since the caliper hole may have contracted from rust. It should be tapped to drill out its threads to fit the new fitting. I'll write this up once I've actually tried it.

Once you've flared and installed a new fitting into the caliper, I recommend spraying the fitting and surrounding caliper body with Fluid Film. This is a waxy coating which resists water. It may help to apply new Fluid Film each year.

[FireFox31]

Reserved for caliper body rust.

[volvolugnut]

This will become an excellent resource.
volvolugnut

[jreed]

Thanks for doing this for us FireFox31... Much appreciated! Going to be replacing rotors and pads on the rear of my '97 855 GLT soon (maybe this weekend) and I'm going to be inspecting the calipers as you suggest.

[scot850]

Will have to check the brakes on the 850 base I am helping with as it has been standing for some time. At least the rear RH parking brake is not working. LH seems to be ok. Based on how easy the rotor turns I am suspicious it is frozen.

Neil.

[wizechatmgr]

One might be able to remove the fitting partially or fully by using a left handed drill bit that is undersized, once the fitting has been removed, clean that out with a tap. It won't make it fully to the bottom due to the flaring that occurs in the caliper for the mating surface of the bleeder.