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Classic Mustangs Tech Forum

Technical discussions specific to 1964-1967, 1968-1970, and 1971-1973 Classic Mustang. Discuss all tech related to in-line six cylinder and V8 powered Vintage Mustangs here.

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Unread 05-19-2011   #1 (permalink)
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Default Front Disk Brakes locking up '66 GT

My '66 GT Conv front disk brakes are locking up when hot. I rebuilt and bled the brakes last July but have not driven the car much since that time. Now when I drive over 1 hour in hot WX the fronts lock up and peddle is up and hard. Fluid is ok. I had to jack up remove the wheels, bleed off pressure and force the calipers back to get a release. Thanks
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Unread 05-19-2011   #2 (permalink)
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Is there pressure in the line between the hose and master cylinder? Is there pressure in the caliper?

Forcing the calipers should do nothing, the 65-67 calipers were fixed. Or do do mean forcing the pads away from the rotor?

What master cylinder do you have?
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Unread 05-19-2011   #3 (permalink)
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Default Front Disk Brakes Locking up '66 GT

Sorry, I meant forcing the pads/pistons back in the calipers. Master Cyl is Ford non boosted. All Stock. New O'rings on pistons and piston cylinders were cleaned and scotched brighted at reassembly.
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Unread 05-19-2011   #4 (permalink)
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Have you changed anything recently? original nonpwr disc brake pedal, new/old MC? Adjustable pedal rod to MC? Did you put new brake hoses on? When rubber hoses get old they can collapse internally causing fluid lock.

It sounds like there is constant pressure on the front brakes and they are overheating. Try driving cold for 5-15 min. then jack up one front tire and see if you can spin it freely or if it sticks.
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Unread 05-19-2011   #5 (permalink)
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jon is right, sounds like hoses.
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Unread 05-19-2011   #6 (permalink)
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Quote:
Originally Posted by Jonk67 View Post
Have you changed anything recently? original nonpwr disc brake pedal, new/old MC? Adjustable pedal rod...
It seems that there is a pre-load on the master cylinder, or even just one of the caliper pistons, that is causing the brakes to be applied a small amount, all the time. The caliper(s) heats up from the constant braking and eventually transfers the heat to the brake fluid. The fluid volume increases with heat and the resulting increase in pressure causes the brakes to apply even more. This progressive failure will eventually lead to brake lock up.
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Unread 05-20-2011   #7 (permalink)
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Degins is right! Check the rod going from the pedal to the master cylinder. If it is not allowing the master cylinder piston to completely release, the front brakes will lock up as the brake fluid heats up. The give away is if when you open either front bleeder, both calipers release allowing you to turn the front wheels.
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Unread 05-22-2011   #8 (permalink)
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Thanks for come backs:
'degins' hit the nail on the head: It seems that there is a pre-load on the master cylinder, or even just one of the caliper pistons, that is causing the brakes to be applied a small amount, all the time. The caliper(s) heats up from the constant braking and eventually transfers the heat to the brake fluid. The fluid volume increases with heat and the resulting increase in pressure causes the brakes to apply even more. This progressive failure will eventually lead to brake lock up.

I rebuilt the clapers/pistons/o'rings/dust covers a few months ago. Yesterday I bled all the old fluid from Mst Cyl (stock Mustang manual master cylinder for disk brakes) it was rust colored and cloudy. Pedal is good with about 3" travel when cold and wheels have slight brake drag when jacked up. After 20 minutes of normal driving at 75'F ambient temp, the pedel gets hard with almost no travel. Both front brakes when measured with an IR temp probe are up to 250'F, back brakes are about 150'F. Wheels are locked up! When the bleed valve is cracked fluid comes out. After about 30 minutes of cooling the pedal gets travel agin and the wheels can be moved with a some force. Brake temp by then is down under 125'F.

Since rust was found in the master cylinder could the problem be with the check valve seal, residual check valve, the piston return spring, primary cup piston or secondary cup? Brake pedal/rod to master cylinder have not been altered.

What about the proportioning valve? It seems to be a little rusty where some brake fluid may have escaped.

Sincerely, Steve
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