Thread: Disc brake drag
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Old 06-28-2011, 08:55 PM   #41 (permalink)
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Join Date: Apr 2011
Location: CA & OR
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Hexd - '93 Honda DX Hatchback
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[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).

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....


Last edited by albyneau; 06-29-2011 at 05:15 PM..
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