Brake fading

Tiny
TKM1951
  • MEMBER
  • 1996 GMC C1500
  • 5.0L
  • V8
  • 2WD
  • AUTOMATIC
  • 123,000 MILES
I changed my master cylinder, while bleeding, snapped bleeders on both front calipers. Installed new calipers. Brakes were soft I couldn’t take it on the road while moving truck in the lane rear brake lines rusted through. Installed new brake lines, two new wheel cylinders.
Now the brakes are softer and fade quickly I have bleed several liters through with no improvement.
Any help or direction would be appreciated.
Regards,
Terry
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Thursday, June 20th, 2019 AT 7:29 AM

7 Replies

Tiny
CARADIODOC
  • EXPERT
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.
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+1
Thursday, June 20th, 2019 AT 3:52 PM
Tiny
TKM1951
  • MEMBER
The master went dry when I lost the rear brake line. Looking back I should have benched the master again. I will give your recommendation a try and report back. This truck does not have anti lock.
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Thursday, June 20th, 2019 AT 4:35 PM
Tiny
CARADIODOC
  • EXPERT
Dandy. Keep me updated on your progress.
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Thursday, June 20th, 2019 AT 6:22 PM
Tiny
TKM1951
  • MEMBER
Okay, I bled from the master as suggested no joy. We made it worse. I said it did not have ABS it does have ABS.
Regards,
Terry
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Saturday, June 22nd, 2019 AT 8:41 AM
Tiny
CARADIODOC
  • EXPERT
If that's four-wheel-anti-lock brakes, you'll likely need a scanner to bleed the hydraulic controller.
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Sunday, June 23rd, 2019 AT 6:35 PM
Tiny
TKM1951
  • MEMBER
How do we determine if it is four wheel anti lock?
Terry
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Sunday, June 23rd, 2019 AT 6:39 PM
Tiny
CARADIODOC
  • EXPERT
Four-wheel systems will have a wire going to a wheel speed sensor at each wheel, except on some GM trucks. To save money. A lot of them use just one sensor in the rear differential, but they still have one on each front wheel. The steel lines leaving the master cylinder will go into a rather large metal assembly with a lot of wires connected to it. That's the hydraulic controller where air gets trapped.

When you have rear-wheel anti-lock brakes, (RWAL), there will be the normal combination valve on the frame rail, right under the master cylinder. The steel lines go into that brass block. From there, the steel line going to the rear brakes will have a "dump" valve along the frame rail. No special bleeding procedures are needed with those dump valves. They're normally open to fluid flow.
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Sunday, June 23rd, 2019 AT 7:06 PM

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