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How to replace a brake line

We crawl under an old grain truck to correct a bad repair 
and properly install a new flexible brake line

This shop class instalment is proof there is some truth in the old expression, “If you want something done right, you’ll have to do it yourself.” That old adage is the reason we’re crawling under this 1960 Chevy grain truck to repair a previous repair.

The truck suffered from brake problems last season, so it was taken to a local garage. After an expensive repair bill, the truck came back, apparently in working order. But after logging less than 150 kilometres, brake problems reoccurred.

The truck was driven into the farm shop for inspection, and in the process of bleeding air out of the lines after refilling the system with fluid, a flexible brake line burst and all pressure was suddenly lost. This truck is from the era when brake systems only had one circuit. If it loses pressure, the truck is immediately left with no brakes at all. Fortunately, this failure happened inside the shop. If the truck was moving down the road, loss of pressure could have caused a catastrophe.

On inspection, it was clear the sudden pressure failure was due to one of several unacceptably poor elements of mechanical work performed during the previous brake overhaul. The line that failed was brand new. Instead of sourcing the right part, the garage jerry-rigged an incompatible replacement. A line that was much too long was installed, curled into a loop and held that way with a plastic zip tie. The line soon rubbed on the wheel rim lip, which wore a hole through the lining. That became the failure point.

So, we stroked that garage off our list of potential service providers and installed the correct replacement line ourselves. Here’s how it should have been done in the first place.

Lessons from the service manual

To guide us through the process and make sure we got it right, we turned to the original GMC shop service manual and followed the recommended procedure to the letter. There’s no room for half measures on a system as important as heavy-truck brakes.

First, installation has to be done with the weight of the truck on the suspension and the wheels facing straight ahead (we’re working on the front axle).

After getting the failed line off, we threaded the new one into the back of the wheel hub. The manual says to lubricate the threads with some brake fluid, first. Once snugged in place there, we slotted the hex section on the other end of the line through the support bracket that holds it to the chassis. This bracket also prevents the line from turning, and it’s important there is no torque or twist on it when slotting it into place in the bracket.

To lock the line in place at the bracket, a spring clip is slotted into a grove in the line’s fitting, holding it secure.

After lubricating the threads on the fitting at the steel line with more brake fluid, it’s torqued into the end of the flex line.

To double check that the replacement line is installed correctly, the manual suggests jacking up the truck and letting the suspension droop to its lowest point. Check to ensure that the new flexible line allows this to happen. If you’re working on the front axle, turn the wheels from lock to lock and make sure the new line doesn’t contact any other component. If it does, it will be necessary to remove the hose end from the support bracket and clock it slightly in one direction or the other to position it differently.

Now it’s time to bleed any air out of the system. Any time you take apart a hydraulic brake system, you’ll have to do this. Air is bled out at each wheel hub. Start at the wheel farthest from the master cylinder and work your way back to the closest. That usually means beginning at the right rear. After that, the left rear. Then, the right front, and finally the left front.

There are speciality tools on the market to make the bleeding process a one-man job. But if you don’t have one, you’ll need a friend to help you. After filling the master cylinder with fluid, have someone push down the brake pedal. While your friend holds it down, crack the bleeder screw on the back of the wheel hub and let a small amount of fluid drain out. Then lock it down again and have your friend let up on the pedal. Repeat this process until fresh fluid drains without spitting air. At that point you’re ready to move to the next wheel and do the same thing.

Be sure to keep an eye on the fluid level in the master cylinder throughout this process. You’ll have to refill it frequently.

Now, the truck is ready to work again. If you want something done right… †

About the author

Contributor

Scott Garvey

Scott Garvey is a freelance writer and video producer. He is also the former machinery editor at Grainews.

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