Brakes gradually lock up while driving

1972 FORD THUNDERBIRD
135,000 MILES • 7.0L • V8 • 2WD • AUTOMATIC
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BEDDIEB64
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My front brakes lock up to the point that they are smoking it seems like after the car has been running around town for 15 minutes the brake pedal gets really hard but will still stop but the brakes are rubbing on the calipers so bad that if I let off the gas the car will come to a complete stop after about twenty feet depending on how fast I am going. The longer I drive, the more the brakes grab until they are locked up. I tried the trick where you pull the vacuum from the booster to check the booster, but they stayed locked up. The pads are new, the calipers were replaced, and I believe the master cylinder was replaced right before I got the car. After the car sits for a while, the brakes eventually release until I drive it for a little bit like 15 minutes or so. Sometimes this starts to happen when I never even hit the brakes. Could it be the booster? The proportioning valve? Is it possible the wrong master cylinder was installed?
Help!
Dec 28, 2021 at 4:49 PM
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JACOBANDNICKOLAS
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Hi,

What is happening is this: The brakes are not fully released. The more you drive, the hotter they get which causes expansion of the brake fluid in the caliper. That makes the pads tighter. As it cools, slowly the caliper releases.

Since this is a1972, it's possible this is a kit to change drum brakes to disc. So, if you know if that is the case, let me know.

First, I need you to check something. If the master cylinder was replaced, they may have incorrectly installed the pushrod causing a small amount of pressure to remain on the plunger. This could cause a slight drag that turns into what you experience. Check under the dash and make sure nothing is preventing the brake pedal from fully returning and make sure the brake light switch doesn't stop it from returning.

If that checks good, I want you to do the following. While the brakes are locked, loosen (not remove) the two bolts holding the brake master cylinder to the brake booster so the MC can move about 1/2" forward away from the booster. Does that allow them to release? If not, move to the next test.

Although the calipers were replaced, there could be a problem with the rubber brake hoses to the caliper. So, when they are locked, very carefully (so you don't get burned) slightly open the bleeder on one of the calipers to see if the brake releases.

Try these things and let me know what you find. Also, if you could take a couple of pictures of the brake master cylinder and the calipers so I can see what I'm dealing with, it would help.

Let me know.

Joe
Dec 28, 2021 at 7:31 PM
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BEDDIEB64
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Okay,

I tried the test of loosening the master cylinder and, if I’m not mistaken, that did not allow the brakes to release. However, I will check that again because since then I personally installed new brake hoses on the front and re-bled the brakes. I changed them because I tried to break the bleeders loose and the brakes released so I thought the hoses might be damaged inside. I can look up brake pads for this car through places like AutoZone and O’Reilly’s, so I believe the front disc are stock however, in 1972 the bolt pattern for wheels is listed everywhere as 5 X 5 but this car has a 5 X 4.5 bolt pattern front and rear so either Ford had some leftover parts since this car was produced in October of 1991 or someone changed things. When I look up rotors and drums, I have to look them up as if it’s a 1971.
That’s pretty much what I’ve tried and what I know at this point but when the rain stops here in Mesa, AZ I will go back to the beginning and try all of the tests and let you know.

Thank you,
Eddie
Dec 28, 2021 at 8:39 PM
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JACOBANDNICKOLAS
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Eddie,

If they released when you opened the bleeder and not when the Master Cylinder was loosened, try opening the steel brake hose at the Master Cylinder to see if that releases them. If that does, the problem is between the hose and Master Cylinder.

Hang in there. We'll get it figured out one way or another. Also, if you can, see if there is a proportioning valve on it.

Take care and I look forward to hearing from you.

Joe
Dec 28, 2021 at 9:09 PM
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BEDDIEB64
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I will try that at the Master Cylinder and yes, there is a proportioning valve.
Dec 28, 2021 at 9:10 PM
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JACOBANDNICKOLAS
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There could be an issue with the proportioning valve. Normally, it can cause the slow release of the rear brakes and not the front. But anything is possible at this point. Make sure there are no steel brake lines that are kinked as well.

I will watch for your reply.

Take care,

Joe
Dec 28, 2021 at 9:17 PM