New member, 1999 Yukon Brake Problems

[ndlight]

I have a 1999 Yukon 4wd with 172,000 miles on it. The truck is totally stock and I am the original owner. My first problem is that when I brake at low speeds the brakes vibrate and then grab hard when almost at a complete stop. Sometimes the problem will lessen or go away for a day or two but will come back. Also, the front driver side brake would sometimes lock up. I first replaced the pads and rotors and checked the wheel speed sensors. Last week, I had the brake lines flushed and the rear brakes adjusted by a mechanic. I got the Yukon back yesterday and the front driver side brake hasn't locked up but the brakes started vibrating again by the end of the day.

There is a hissing noise(vacuum leak?) coming from under the dash. The noise starts a few seconds after the truck is started and stays the same at any RPM. I disconnected the brake booster and plugged the hose and the noise didn't change. What other things run off of a vacuum?

Any help would be appreciated.

 

[SunlitComet]

No other vacuum devices are in the cab. Make sure you pump the brakes after you disconnect the booster hose as it retains some vacuum. See if the noise stops them. How long from rotor and pad replacement to them acting up again?

 

[kcb37]

Do you mean the brake pedal vibrates?
If so it is abs kicking in.

When you check the wheel sensors, clean the mounting surface and the sensor.
Sometimes they get rust/corrosion under them and lift, creating a false signal, and activating the abs at low speed.

 

[ndlight]

After I disconnected the brake booster I pumped the brakes before starting the truck and while it was running and the noise is still the same. After I changed the rotors and pads it was acting up within a week. The brake pedal doesn't really vibrate. The whole truck lightly jerks like it would if I pumped the brakes while stopping. I'll check the wheel speed sensors again. Thanks.

 

[SunlitComet]

jerking side to side?

 

[kcb37]

So it's like the brakes grab release, grab release.
Take it you did not change the brake hoses?
Rubber brake line from frame to caliper.

Not something that happens everyday kinda thing. But when they get old, they can "fall apart on the inside" When this happens they can create a one way valve.
Meaning you can hit the brake and have no left front brake because the fluid cannot reach the caliper.
Or the other way around, hit the brakes full pressure full brakes. Release the brakes and the left front does not.


No clunking, doesn't feel weird when driving at all?
Thinking a brake hose may be possible but if your steady on the brake pedal it should not do that.

Did you look at the rear brakes?
Not contaminated are they?
Cleaned off the front rotors before you installed them?
Nothing dripping on the brakes/splashing up other then water from the road etc...?

 

[ndlight]

Jerking back and forth like the grab release kcb37 is talking about. I did clean the rotors before I installed them and there is nothing dripping on them. It drives fine and I don't feel it while braking at high speeds. In Sep. '09 both front calipers, both rear drums, shoes, and cylinders were replaced by a mechanic. I'm assuming he replaced the rubber hoses when he replaced the calipers. I did look at the rear brakes and they look fine. I checked the front wheel sensors with my meter and both read about 250v when spinning the tires by hand. I read on a post that if it reads below 350v there is an improper air gap and they need to be replaced. Does that sound right? Thanks.

 

[SunlitComet]

they should be at least 350ACmv's or higher. between that and 200 it might mean that there is debris or corrosion on the heads or reluctor.

 

[ndlight]

I'll have to check it tomorrow. Thanks for your help.