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Replaced 1978 cj7 original brakes with vacuum brakes. After

Customer Question
driving 15-20 minutes brakes will...
Replaced 1978 cj7 original brakes with vacuum brakes. After driving 15-20 minutes brakes will start applying themselves. Brakes will eventually lock up.
Submitted: 1 year ago.Category: Jeep
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Answered in 16 hours by:
7/2/2016
Jeep Mechanic: K. Wiggins, ASE Certified Technician replied 1 year ago
K. Wiggins
K. Wiggins, ASE Certified Technician
Category: Jeep
Satisfied Customers: 3,132
Experience: ASE Certified Technician, Associates Degree in Automotive Technology, 15+ years in the trade
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Hello, are you running a check valve on the booster hose? Did you replace the master cylinder as well?

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Customer reply replied 1 year ago
I installed a new master cylinder. Also I have a check valve installed that was supplied with the other parts.
Jeep Mechanic: sprinkles08, ASE Certified Technician replied 1 year ago
sprinkles08
sprinkles08, ASE Certified Technician
Category: Jeep
Satisfied Customers: 23,526
Experience: Chrysler/Dodge/Jeep Master Certified, ASE Master and Advanced Certified, Trans and Hybrid Specialist
Verified

Hello and welcome to JustAnswer!

You most likely have a bad booster. What you'll want to do is loosen the master cylinder nuts a few turns when the brakes lock up and see if they release. If so then you'll need to be replace the booster.

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Customer reply replied 1 year ago
I retested the Jeep today. If I leave the vehicle just sit and idle the brakes set up without depressing the pedal in 15 minutes. Once the vehicle sits for 1/2 hour or so the brakes are released. I tried this with the vacuum line hooked to the booster and also with it removed same result. Is it possible that the brake fluid is getting so hot it could set up the brakes? I have moved the master cylinder forward because of the installation of the booster. This jeep is running a Chevy 350 and the intake manifold pressure is between 18-20 pounds.
Customer reply replied 1 year ago
One other comment. The lines that went to the original master were used. I needed to stretch them out a bit to fit. This took some of the coil out of the lines but they are not crimped or bent. Could this have caused a problem?
Jeep Mechanic: sprinkles08, ASE Certified Technician replied 1 year ago

Did it have hyroboost on it before or was it a manual system?

Where did the parts come from?

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Customer reply replied 1 year ago
It did not have hydroboost before I put it on. The parts were bought from Summit racing
Jeep Mechanic: sprinkles08, ASE Certified Technician replied 1 year ago

Was it a kit or did you buy individual parts? Can you supply the part numbers of what was installed?

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Customer reply replied 1 year ago
SUM 760-210 kit
Jeep Mechanic: sprinkles08, ASE Certified Technician replied 1 year ago

Are all four wheels locking up?

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Jeep Mechanic: sprinkles08, ASE Certified Technician replied 1 year ago

Do you have a proportioning valve?

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Jeep Mechanic: sprinkles08, ASE Certified Technician replied 1 year ago

Is it new?

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Customer reply replied 1 year ago
No
Customer reply replied 1 year ago
Another idea came up today that the proportioning valve and brake lines could be to close to the exhaust. Have you ever seen brakes fluid get so warm that the brake fluid boiled or expanded enough to press the brakes calipers in.
Customer reply replied 1 year ago
Another thought, let's say the brake fluid is getting so hot it's expanding and pushing the calipers in. If I left the cap off the master cylinder would that allow the brake fluid to expand back into the master instead of the calipers?
Jeep Mechanic: sprinkles08, ASE Certified Technician replied 1 year ago

Have you tried loosening the master cylinder from the booster to see if the brakes released?

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Customer reply replied 1 year ago
it did not release the brakes
Jeep Mechanic: sprinkles08, ASE Certified Technician replied 1 year ago

If you loosen the lines at the master cylinder will they release?

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sprinkles08
sprinkles08
sprinkles08, ASE Certified Technician
Category: Jeep
Satisfied Customers: 23,526
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