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Old 11-27-2013, 05:53 PM
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1993 Merc 300D brakes seizing.

I am really baffled as well as 3 other mechanics at what could be causing my brakes to seize up like they are doing. It started when I changed my master cylinder and brake booster. But since then, my calipers have been seizing in an alternating fashion. The front left brake seizes one time, then the front right, and then the back calipers seize all after just a short time of driving. If I disable the booster, then the brakes don't seem to have a problem with seizing, and I have already changed the booster twice thinking I just received a defective one. I have also changed the brake lines, the front caliper twice before I realized that it wasn't the only one seizing and the ABS pump. In fact, one of the mechanics completely went around the ABS module and the problem still persisted.

Any help would be much appreciated.
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Old 11-27-2013, 07:49 PM
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Join Date: Mar 2006
Location: Mount Holly, NC
Posts: 26,247
What brand MC? The MC has direct lines to left, right, and rear... so gunk or defective MC will cause issues ... also, did you replace the rubber lines?
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Old 11-27-2013, 08:33 PM
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Location: Long Beach,CA
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I can't explain the intermittent problem.

If it was a W123
But, inside of the Master cylinder you have return ports. If any thing blocks the a port the Fluid cannot return to the Master Cylinder and it remains under pressure.
It could be something inside floating around blocking or there could be some dimensional issue with the Rod Length verses the Brake Booster that is keeping the Piston in the wrong position and the return port obstructed.
Or perhaps the Booster is not retracting enough when you release the Brakes.

The old Brake Hoses can act like a one way valve if the insides are messed up.

If a Caliper is frozen and you loosen the line at the Master Cylinder and Fluid spurts out when you loosen the Fitting it is a problem in the Master Cylinder or Booster.

If it only spurts when you loosen the Bleeder Screw it is a problem inside of the Brake Hose.

Some Master Cylinders have 2 Pistons that are not connected to each other except that on piston pushes on a Spring and that other piston pushes on anther Spring. It is sometimes possible for only one of the Pistons to stick or delay returning.
84 300D, 82 Volvo 244Gl Diesel
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Old 11-28-2013, 10:24 PM
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Join Date: Oct 2012
Posts: 4,502
To me it sounds like the anti lock braking system is acting up. May want to look at a factory manual to find out if there is a special way to make sure it is bled down properly.

My feeling is that this ransom wheel application of brakes being different wheels and not in pairs. Have you tried driving with the control module unplugged yet. That's if it has one and just leaves you with normal power brakes as a diagnostic aid.

This is just a guess by me and remember I cannot even guess the winning super lottery numbers in advance. So take my thoughts with a grain of sand.
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