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MG MGB Technical - Brake Caliper Dragging

I have a minor dragging problem with my left front caliper. If I jack up the front end and spin the wheel it rotates about one full turn or less and then comes to a stop. The right side will rotate about 2 or more turns before it comes to a stop.

I checked the bearings with the caliper removed and they are adjusted properly as the wheel rotates as it should. I think that the problem is in the caliper itself, so I am planning to take it apart and install new seals. But before doing so, I plan to check the brake line for an obstruction as this could be the problem. If the brake line supplying that caliper has an obstruction that prevents all of the brake fluid from returning to the master cylinder it could create a slight residual pressure that causes the caliper to not fully release thus causing the drag. I plan on testing for an obstruction by:

1. Placing a clear piece of tubing, about 2 feet long, on the bleed nipple and holding it in a near vertical position.
2. Open the bleed nipple.
3. Have an assistant slowly depress the brake pedal and observe the fluid rise in the tube.
4. Have the assistant slowly release the brake pedal and observe what happens.

If my theory is correct, all of the fluid in the tube should drain back into the caliper and master cylinder if the line is not obstructed. My question is: Is this a good test to check for an obstruction in the brake line? If not, what is a better test?

Any and all advise will be appreciated.

Frank Grimaldi
Frank Grimaldi

Only a certain amount will drain back. As the MCyl moves back it will uncover the feed from the reservoir and as this is higher than the caliper the pressure will be higher (marginally but enough) at this point to stop the back flow. After all this the basic principle of operation as pads/shoes wear.
Also, check for run out on the disc as this will cause the same symptoms.

M McAndrew

Frank. Agree with Mike, especially about checking disc runout. I have seen a single hub which had excessive run out and turning the disc had no effect on correcting the problem. A dial indicator on the disc, then rotate it a complete revolution, will show if/how much runout is present. If it is present on the disc, it is worth checking the runout on the hub itself. The friend's car had the problem corrected by turning the disc while mounted on the hub, truing the disc to the hub and making it true to the caliper. Since, it would seem, that Mike has seen this problem at least once, and I have seen it once, it is worth checking as part of your testing process.

Les Bengtson

Sounds more like runout to me, if the calliper were still partially pressurised the wheel is unlikely to move very far at all and would be dragging all the way.

I doubt any would run back. The master seals are very unwilling to pull fluid back, their design means that with the slightest resistance fluid is moved from behind the main seal to in front of it. Remember there are two seals in the master - the main pressure seal that starts off behind the port from the reservoir, then closes it off as soon as the pedal starts moving, and the secondary seal which is always behind the port. The space between the two seals is filled with fluid. With the bleed nipple open and the pedal fully released the fluid will try and find its own level i.e. that in the tube would rise to that in the master. To fall the level in the tube would have to be higher than the master.

You didn't need to remove the caliper, if fact with it in place it should be much easier to gauge any runout of the disc. All you need to do is try and push each pad/piston back in turn. If the first is very hard, and the second simply pushes the first back out again, then a blocked or partially blocked flex hose at the caliper is implied.

Perhaps even simpler is to apply the brakes hard, release, then immediately open the nipple, which will release any retained pressure. If the wheel was locked before you opened the nipple, and released afterwards, then the fluid path is blocked somewhere.
PaulH Solihull

If you are still on fairly old rubber flexibles, change them first, I've known them to lock the wheel completely. If there is debris in the fluid when bleeding,that's a fair indication that the flexibles are deteriorating but age ought to be the sole determining factor!! I bit of "run out" is tolerable, in fact it should increase pad/disc clearance. Also check that the pad/caliper interface is clean, rust free and smeared with a little copper slip.
Your theory is flawed, and not to be recommended! Only a tiny amount of fluid is moved under normal operation. Your friend would push more fluid through than that. There's also the chance of letting air in.
Allan Reeling

This thread was discussed between 23/01/2012 and 24/01/2012

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