Stuck Caliper on '97 Accord

[poppitb]

OK, so I went on vacation to the shore 600 miles away and that was probably my first mistake. My son calls and says he heard a nasty "crack" sound and then had no brake pressure in his '97 Accord. It came back and when I got home and checked it it seems fine. The problem is his left front caliper is stuck on tight. Now I've fixed lots of brakes but never had a stuck caliper like this one.
Approaches? My gut says open the bleeder and see what happens. If it shoots fluid and releases I'd guess the caliper is fine and the problem is elsewhere. If it doesn't is where I'm concerned. We were about to try it just now (had the wheel off in the road) but a thunderstorm rolled in and killed that idea. What to do if it doesn't release?
And if it does, does that mean I have a clog in the line somewhere?
Thanks!
Tim

[TheBobsHere]

The caliper wont just retract, you need to put pressure on it to get it to retract the piston. If you cant get it to retract then you may have a bad caliper, but it sounds like it could be the master cylinder. You'll also need to bleed the system after opening the bleed screw. Any fluid leaking anywhere?

[mpumas]

The caliper piston should retract to a relaxed position (brake pad not touching the rotor) when the pressure is taken off the brake line, however, if the caliper pins are stuck, the piston will retract but the caliper assembly will not reposition itself to the relaxed position thus moving the brake pad on the opposite side to move away from the rotor. If you have a problem with just one caliper on a car, it is not the master cylinder. Bleeed the caliper, then see if you can move the caliper assembly back and forth. If not, you need to pull the caliper pins and hone out the rust, lubricate with silcone grease, clean or replace and lubricate the pins. Or you could have a broken caliper assembly.

[TheBobsHere]

The calipers retract by negative hydraulic pressure. If the MC is bad as I suggested or there is air or a leak in the line the caliper will not retract on its own. The comment "then had no brake pressure" to me suggests that it was a system wide failure.

[TheBobsHere]

The pin may be a problem but that would mean the bolt broke allowing excessive pedal travel described (again the comment "then had no brake pressure") and would be easy to find and solve. There's no way of telling until the caliper and system are thoroughly checked.

[mpumas]

There are pressure and no pressure conditions in the brake system. I don't think there is ever negative pressure (vacum). The piston retracts due to a sealing O ring on the piston that does not normally move when the piston is moved out when the brakes are applied. When the brakes are released, this O ring pulls the piston back enough so the pad doesn't touch the rotor. Releasing the brakes doesn't pull the piston all the way back into the caliper assembly which negative pressure in the system would. That is the self adjusting feature of the front disc brakes.

[TheBobsHere]

The master cylinder is responsible for pushing the piston out for stopping then pulling the piston in just enough to not contact the rotor on disk brakes and a spring is responsible for retracting the pad and piston on drum brakes. How does an O ring move anything? It's a seal.

[TheBobsHere]

As I vaguely explained above, I've also seen guide pins get cruddy and bind or seize, thus causing the caliper to put pressure on the piston side of the rotor only. Then suddenly one day the pins come free or the bolt breaks (as I described above) and cause excessive pedal travel for a very short time (until the caliper re-seats itself) then the pedal pressure returns to normal. This uneven pressure condition can do damage to the wheel bearings, sometimes causing a loud crack sound and lock up of the wheel due to broken wheel bearings. Seized guide pin conditions can be identified by unevenly worn pads (the piston side pad is thinner than the its twin). As far as identifying wheel bearing lockup that's easy, if the the caliper isn't binding the rotor and the wheel doesn't spin freely (or at all) the wheel bearing is most likely bad.

Just though my comment above needed a little more explanation.

[poppitb]

Ohhh uglier and uglier the deeper into the pit we go....
Open the bleeder and no pressure, just a little weeping. Pump some fluid through and its like the day it was put in - clear with no particulates.
Removed the lower guide and shockingly the caliper swung right up.
Outer pad worn, but still has 5/16ths on it. Check.
Inner pad has....wait - no inner pad! Gone, as in no pad material, backing plate, nothing. No pad at all, caliper piston against the rotor.
Ug-glee.
Today we get pads and pull the rotor off and try to retract the piston. Bent the handle on my 6" C-clamp trying to move it. Even with the bleeder open. OK, call NAPA and they can have one tomorrow. $53 with a $83 core charge. GO out and pull the old one off (in the rain) and notice the inner rotor is waayyyy thinner than the outer. Ut-oh. Seems I read the rotors are a class A P_I_A on this car. My mechanic friend said don't even try. Might after I read up on them tonight.

More to come...
Thanks for playing!
Tim

[TheBobsHere]

Unless you're pressing the pedal there wouldn't be any pressure. That crack sound your son heard was the pad leaving through the gap between the rotor and caliper and hitting stuff on it's way to it's new home on the streets, the lack of pressure was the piston filling the space the pad used to occupy, replace the caliper and change all of the pads on the car for good measure. Pay special attention to the guide pins and make sure they're in good condition and not binding. As far as replacing the rotor it should be four bolts accessible from the front of the rotor. You might want to have the bearings inspected and repacked (if they're blue or purple replace them). As long as you take it easy you can get by until you get the rotor fixed.