Disc brake drag

[albyneau]

[QUOTE=MazdaMatt;110029]disc brakes only push against the disc, they don't pull away. generally they are bumped back on their own. Two things can make them not bump back - your wheel bearings show zero flex (good) and your discs are perfect (good), or your calipers aren't sliding well (not bad) or stuck (bad).
QUOTE]

The predominance of my time here's spent quietly lurking in the corner~ but occasionally I find the need to pipe in. This is one of those times.

Discs are returned to near contact-free status by two means; the aforementioned square o-ring, which sits in a taper-cut groove and does indeed PULL the piston back a few thousandths. The second and most important is by intentional design, and must not be altered~ and that is the fact that front wheel bearings do have, AND NEED, some play in them. The tapered rollers (or balls if your bearings are really, REALLY old) need to lose contact with the race for two reasons of their own; to allow lubricant flow, and to cool off. The ONLY exception to this is a non-automotive, low-RPM, oilbath application. This is why when repacking your bearings you back the nut off just a bit (check your FSM for your specs)~FWD's have this clearance built into their preassembled hubs. With this mandatory lube clearance comes the fringe benefit of that ever so slightly wobbling rotor physically knocking the caliper away just enough to prevent excessive rubbing/wear/noise.

What is too much drag? You should be able to jack the vehicle up, spin the wheel as hard as you can, and on drive axles (F or R) have it freewheel on it's own (in neutral, obviously) at least one full rpm~ due to drivetrain drag. Front discs on a rear-drive should spin several times~ making only the slightest of hissing/drag noise. If not, then there's too much drag. Caliper o-rings rarely fail, unless quite old, the outer dustboot is failed/damaged, and/or they've seen excessive heat. Look first at the caliper sliders~ they're the likely culprit, hanging out in the air/dirt/roadkill/water stream ever flowing under your ride.

Now, that darkly discolored fluid mentioned earlier? You've no idea how important that is! I've been wrenching for nearly half a century now (started when I was 2 ) and I've met so few mechanics and even instructors that ever heard this one. It's a story akin to diamonds~ of pressure and time. That darkness you see there is decomposing rubber from the brake system! But how could that be? Brake fluid doesn't dissolve rubber! Over time the brake fuild absorbs water from the air. During the life of your brakes under pressure and heat this fluid/water mix is chemically altered, and turns into AMMONIA~ and it is this ammonia that eats up your brake seals. This is why it's imperative to replace your brake fluid every three years, or 20-30K miles~ whichever's first. Rarely will you see a FSM/owner's manual even mention this. This is referring to DOT-3/4 fluids~ DOT-5 is a different beast....

Check valves, or more accurately residual valves, are far more common than you can imagine. Common are 2-3psi on discs, and 10-13psi on drums. They're hidden in your master cylinder....

Peace~
Scott

[Old Mechanic]

Mercedes even has service bulletins for the parts department on how to store brake rotors to keep them from warping before they were installed as replacements.

The previous post is precisely correct, concerning newer calipers and their ability to retract the pads a few thousandths to minimise drag. It does require rotors to be smooth and true to work properly. It will still work with worn-warped rotors but nowhere near as well. We turned every rotor before installation to make sure things started out with everything smooth. I have returned new rotors that were warped 40 thousandths right out of the box. possibly the improper storage MB had warned us of decades earlier.

On my 94 VX the brake fluid was so bad after sitting for 15 years the rubber under the cap on the master cylinder reservoir had basically disintegrated. Change the fluid as previously recommended, but I have some more suggestions based on experience.

If your car is over 10 years old, you may loose the master cylinder if you pressure bleed and push the pedal to the floor. It is better to flush the system while not pushing the pedal any further than its normal travel. If you suspect dragging or sticking calipers then you can diagnose things fairly quickly with a digital thermometer (also useful for a lot of other diagnostic procedures). Do a test drive and just read the temp at each wheel. If one is much hotter or colder than the others then you can safely assume the issue lies there.

regards
Mech

[Frank Lee]

Yeesh, talk about disc brake drag- I took a car of mine that I hadn't driven in 3 years on a long trip and discovered the right front caliper didn't want to release. I had to pull over 4x in 350 miles to jack up the front, remove the wheel, and pry the caliper open with the tire iron. Thanks to a tailwind and slow 'n' steady driving I still got 40.2 mpg, including some city driving where I'd made a few wrong turns.

Bought some tools and honed it out when I got there. Thought I'd be golden for the trip back but noooo. This time after prying the caliper open I went 250 miles to my destination without using the brakes at all via judicious use of ignition-off downshifting!

I've read about rubber lines collapsing internally but hadn't had that happen to me before. Looks like it's gonna get new rubber lines next...

[momorris]

Been having a nightmare with brake drag this weekend. Cleaned sliders, changed hose. At the moment it looks like the internal piston seal has swollen in the caliper that was dragging.

I swopped the pistons and seals over from passenger to drivers and vice versa to see if the problem also moved. While doing this I compared the seals and one was definately larger. Doing some more research it could have been caused by the garage using coppergrease instead of silicone or red brake grease.

[Old Mechanic]

Originally Posted by Frank Lee

Yeesh, talk about disc brake drag- I took a car of mine that I hadn't driven in 3 years on a long trip and discovered the right front caliper didn't want to release. I had to pull over 4x in 350 miles to jack up the front, remove the wheel, and pry the caliper open with the tire iron. Thanks to a tailwind and slow 'n' steady driving I still got 40.2 mpg, including some city driving where I'd made a few wrong turns.

Bought some tools and honed it out when I got there. Thought I'd be golden for the trip back but noooo. This time after prying the caliper open I went 250 miles to my destination without using the brakes at all via judicious use of ignition-off downshifting!

I've read about rubber lines collapsing internally but hadn't had that happen to me before. Looks like it's gonna get new rubber lines next...

Frank if the rubber lines are swollen internally you will get a very slight pffft of fluid if you open the bleeder fairly quickly. You might even be able to hear it unless you are as deaf as I am now. If the brake is dragging and you pop the bleeder and it stops dragging, until you hit the brakes again, it's also a pretty good indication the line is bad.

regards
Mech

[shorttimer]

Funny I found this particular thread on brake drag. I recently replaced the calipers on my van because they were dragging so bad. It had set for a long time & hadn't had the proper maintenance. After the change they worked as new. It's my only 4 wheeled vehicle. Since using small displacement motorcycles & mopeds, for the last 10 years, I rarely use it. But here's what I'm actually getting at: brake systems are the most under maintained systems on a vehicle, on average. The changing/flushing of fluid is something that rarely, if ever, gets done. On top of that the calipers are rarely cleaned, which is important for the reason this thread was started in the first place. Unfortunately it's a lot of work on a car, opposed to a bike. What needs to be done is to disassemble the caliper, take out the pucks n seals n dust covers. Clean them spotless, including the groove the seal rests in. Sometimes it requires a pointed scraping tool to get into the groove. I have a bunch of stainless steel dental tools which work quite good for that. While you're at it, inspect the puck & rubbers for any damage & obviously replace if worn. Especially the seals. When reassembling, apply brake fluid to all the parts so they slip together easily. You will be amazed at how much better they work. It's very noticeable on motorcycles, too. Hope this helps.