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sprinkles08
sprinkles08, ASE Certified Technician
Category: Jeep
Satisfied Customers: 21968
Experience:  Chrysler/Dodge/Jeep Master Certified, ASE Master and Advanced Certified, Trans and Hybrid Specialist
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Replaced 1978 cj7 original brakes with vacuum brakes. After

Customer Question

Replaced 1978 cj7 original brakes with vacuum brakes. After driving 15-20 minutes brakes will start applying themselves. Brakes will eventually lock up.
Submitted: 7 months ago.
Category: Jeep
Expert:  K. Wiggins replied 7 months ago.

Hello, are you running a check valve on the booster hose? Did you replace the master cylinder as well?

Customer: replied 7 months ago.
I installed a new master cylinder. Also I have a check valve installed that was supplied with the other parts.
Expert:  sprinkles08 replied 7 months ago.

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.

Customer: replied 7 months 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: replied 7 months 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?
Expert:  sprinkles08 replied 7 months ago.

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

Where did the parts come from?

Customer: replied 7 months ago.
It did not have hydroboost before I put it on. The parts were bought from Summit racing
Expert:  sprinkles08 replied 7 months ago.

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

Customer: replied 7 months ago.
SUM 760-210 kit
Expert:  sprinkles08 replied 7 months ago.

Are all four wheels locking up?

Customer: replied 7 months ago.
Expert:  sprinkles08 replied 7 months ago.

Do you have a proportioning valve?

Customer: replied 7 months ago.
Expert:  sprinkles08 replied 7 months ago.

Is it new?

Customer: replied 7 months ago.
No
Customer: replied 7 months 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: replied 7 months 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?
Expert:  sprinkles08 replied 7 months ago.

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

Customer: replied 7 months ago.
it did not release the brakes
Expert:  sprinkles08 replied 7 months ago.

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

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