If the truck has anti-lock brakes, you will likely need a scanner to command the computer to open two valves so those chambers can be bled. This only applies if the master cylinder was allowed to run empty. That lets air get into the hydraulic controller. Once set up, this only takes less than half a minute.
GM doesn't have much trouble with front calipers, so the next time you break a bleeder screw, rather than replacing a good caliper, you can loosen the banjo bolt that holds the rubber flex hose to the caliper, remove the caliper from its mount, hold it so that hose connection is at the high point, then loosen the bolt some more to bleed the air out. We used to drill out the screws, tap the holes, then install new screws or over-sized repair kits, but today professionally-rebuilt calipers are so inexpensive, it's hard to justify the time it takes to do the repair that way.
Here's another trick when you're replacing a master cylinder. When you replace the master cylinder with two steel lines, loosen the line nuts a little, remove the mounting bolts to the power booster, pull the master cylinder forward, then use it as a handle to bend the steel lines up a little. That will keep the fluid from running out of the lines.
Remove the two lines all the way, then remove the master cylinder. Brake fluid eats paint, so be careful to not allow any to drip onto the car.
Screw the two lines into the new master cylinder that has been bench-bled, then use it to bend those lines back down to their normal shape. Bolt it to the booster, then snug one of the line nuts. Have a helper slowly push the brake pedal half way to the floor. It should take about 15 seconds to do that. You'll see bubbles coming out by that nut. Snug the nut, then holler to the helper to quickly release the pedal.
Do that a second time, and perhaps a third time, until you see only clear fluid with no bubbles coming out, then do that for the other line. By pushing slowly, fluid will get pushed down the lines, and air will float back up. By releasing the pedal quickly, the fluid rushing back will wash the air back up into the reservoir with it. This can even work when working on the car by yourself, just keep the line nuts tight.
This wondrous trick might not work well on Fords that have four lines at the master cylinder, if you want to release the car to a customer. It still works when you drive the car normally yourself. With every brake pedal application at a red light, a little more air will make its way back up into the reservoir. Within a few minutes the brake pedal will feel normal.
Thursday, June 20th, 2019 AT 3:52 PM