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Old 07-31-2006, 12:15 PM   #1
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Two years ago I replaced my hoses & calipers because the calipers were hanging up, causing my tires to overheat and blister. In one case, the rim got so hot that my valve stem started to melt.

This past weekend I just had a tire problem again. The front tires were replaced two years ago after I replaced the calipers, hoses, and brake fluid. Last friday I took a 200 mile ride. When I got to my destination I notice the front left tire was very low. It was fine when I left. I discovered that the sidewall has damage and a hole which looks like it was caused by excessive heat buildup. The tire is an Aurora and has been kept at 80PSI.

I wonder if my relatively new calipers are hanging up. With the vehicle jacked up I can turn the hub with my hands after the brakes are released. It does take some amount of force to turn the rotor, but I do not know if it is a normal amount of force or not.

Question 1: On a scale of zero to ten, how much force should I need to apply to get the hub to turn after releasing the brake? Zero = freewheeling and ten = totally locked up.

Question 2: Should I assume my calipers are fine if on a level road at idle the RV will move forward when the brakes are released? (when I was having the problem two years ago, the RV would not move if I released the brakes)

I'm taking the tire back to the dealer tomorrow to see what they have to say about it.

Thanks,
Rob
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Old 07-31-2006, 12:15 PM   #2
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Two years ago I replaced my hoses & calipers because the calipers were hanging up, causing my tires to overheat and blister. In one case, the rim got so hot that my valve stem started to melt.

This past weekend I just had a tire problem again. The front tires were replaced two years ago after I replaced the calipers, hoses, and brake fluid. Last friday I took a 200 mile ride. When I got to my destination I notice the front left tire was very low. It was fine when I left. I discovered that the sidewall has damage and a hole which looks like it was caused by excessive heat buildup. The tire is an Aurora and has been kept at 80PSI.

I wonder if my relatively new calipers are hanging up. With the vehicle jacked up I can turn the hub with my hands after the brakes are released. It does take some amount of force to turn the rotor, but I do not know if it is a normal amount of force or not.

Question 1: On a scale of zero to ten, how much force should I need to apply to get the hub to turn after releasing the brake? Zero = freewheeling and ten = totally locked up.

Question 2: Should I assume my calipers are fine if on a level road at idle the RV will move forward when the brakes are released? (when I was having the problem two years ago, the RV would not move if I released the brakes)

I'm taking the tire back to the dealer tomorrow to see what they have to say about it.

Thanks,
Rob
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Old 07-31-2006, 06:42 PM   #3
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Hanging calipers can be caused by other things. In hydraulic brakes, a damaged hose can act like a check valve and keep the brakes from dragging.

If you experience dragging in both front calipers, you may want to take a look at the master cylinder and the power booster pushrod adjustment.

There is a small hole in the master cylinder bore called the bypass port. When the brakes are at rest, the bypass port is directly in front on the main master cylinder piston seal. If the master cylinder pushrod is too long (most vacuum boosters have an adjustable pushrod), the bypass port is covered up.

When the brake fluid heats due to use, it expands. Normally, the excess fluid passes into the master cylender reservoir. If the bypass prot is covered, it cannot do this. The result is the brakes applying. This creates even more heat.

Also, master cylinders manufactured for use with drum brakes sometimes have check valves in the line. If used with a disc brake, the check valve will keep the brakes slightly applied which will also cause the heat.

Has the master cylinder been replaced or serviced?
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Old 08-01-2006, 05:36 AM   #4
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As far as I know, the master cylinder has never been replaced or serviced. The RV had 12K miles on it when I got it four years ago. Now it has about 39K. It is 31ft long class A on a '93 Ford F53 chassis. It has disc brakes front and rear.

I did not know that there was any adjustment for the pushrod. I will look into that. When I was having problems two years ago, it seemed that the brakes would release a little better if I actually put my foot underneath the brake pedal and pulled up on it. Does that indicate that it could be related to this bypass port?

I've replaced my calipers and hoses myself without much difficulty. Is it easy to replace the master cylinder? Are they expensive?

Thanks.
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Old 08-01-2006, 10:31 AM   #5
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<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Rob Hentges:
As far as I know, the master cylinder has never been replaced or serviced. The RV had 12K miles on it when I got it four years ago. Now it has about 39K. It is 31ft long class A on a '93 Ford F53 chassis. It has disc brakes front and rear.

I did not know that there was any adjustment for the pushrod. I will look into that. When I was having problems two years ago, it seemed that the brakes would release a little better if I actually put my foot underneath the brake pedal and pulled up on it. Does that indicate that it could be related to this bypass port?

I've replaced my calipers and hoses myself without much difficulty. Is it easy to replace the master cylinder? Are they expensive?

Thanks. </div></BLOCKQUOTE>

Don't forget to check for tight or hanging mechanical linkage connections betwween brake pedal and booster unit. May require lubrication.
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Old 08-01-2006, 06:34 PM   #6
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You can test the master cylinder by removing the reservoir cover. While looking into the reservoir, have an assistant press the brake pedal. If the pushrod is adjusted properly, you will see a slighbt disturbance in the fluid. If you don;t see anything, the bypass port is not being uncovered when the brake pedal is released.

Be careful. Brake fluid is one of the best paint removers made.
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Old 08-02-2006, 08:56 AM   #7
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<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by mrschwarz:
You can test the master cylinder by removing the reservoir cover. While looking into the reservoir, have an assistant press the brake pedal. If the pushrod is adjusted properly, you will see a slighbt disturbance in the fluid. If you don;t see anything, the bypass port is not being uncovered when the brake pedal is released.

Be careful. Brake fluid is one of the best paint removers made. </div></BLOCKQUOTE>

Amen to that!!!!
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Old 08-02-2006, 04:29 PM   #8
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I have had a similar experience in the past. In my case and it sounds like yours, only one wheel out of four had the heat issue.
The problem turned out to be a piece on the brake hose had deteriated and clogged the line. When the pedal was pressed it pressurized the line and pushed the debris in the line. However it would not let the fluid backflow when the pedal was released to depressurize the line. I took it in and had the flex lines replace and all the metal lines blown out and cleaned...PROBLEM SOLVED...
I hope this helps a good luck.
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