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Weak brakes

I have a 1999 GMC 4x4 1500 pickup. The brakes are weak. The pedal will easily go to the floor. If I’m on ice or snow, the anti-lock system will work, but not on dry roads… it won’t even come close to skidding a wheel and activating the anti-lock system. I’ve replaced the master cylinder, blead the lines, replace the brain box (I think that’s what it’s called), new pads & rotors. There’s one more part to change, I think, but it’s expensive, so I’d like to know if that is likely the problem.


Chris in Estes Park, CO

If the pedal will easily go to the floor, you most likely have air in the system. Did you bench bleed the master cylinder before you installed it? Why did you replace all these parts? Did you have an ABS code indicating failure of the brake controller, hence replacing the “brain box?” I would like to know what this “one more part” to change is, since there are many parts you do not list as having been changed. What exactly prompted you to replace all these parts in the first place?

It does feel like there is air in the system. The master cylinder was bench bled. I did have a computer code saying the brain box/pump was bad. After I changed that, the code went away. I then bled the brakes and got a hard pedal, but after I started the engine, the pedal went soft again immediatly. I talked to a parts guy and her said it’s probably a valve in a valve body stuck open.

Edit: I do most of my mechanicing, and am usully correct in figuring out what the problem is, so that’s why I changed those parts.


I’m still curious as to what this “one more part” is, but I do have a suggestion for a possible cause of your spongy pedal. On Chevy trucks of this era, the rear disc brake pads often fit very tight in the hardware. Sometimes so tight you had to beat the pads into the hardware and anchor with a hammer. When parts fit this tight, the pressure of the caliper piston can’t even push the pads into the rotor face. This is a common cause of the rusty rear rotor syndrome on these trucks, and can certainly cause a spongy brake pedal due to the extra work having to be done by the front brakes to pick up the slack. I normally wouldn’t condone modifying brand new parts, but I have found that if you grind down the tabs on the brake pads, just enough to get a more normal fit where they slide with minimal resistance, the rear rotors last longer without pitting and the pedal feels a lot better. Grind the tab and around the tab, test fit, repeat until they install like a normal set of brake pads on any other vehicle. Just remember not to grind too much. You can always take more off, but you can’t put it back on once it’s off.

Thanks for your help so far. I sent an email off to find out what that part is called. I’ll post it here when I find out. I didn’t have to force any parts back tegether when I changed out my pads and rotors… but Saturday I will take the pads out and make sure nothing is binding anywhere.

Do you have an idea why the brakes pumped up to a hard pedal when I bled them, then went soft again after I fired the engine?


The power assist will make problems more apparent. I have experienced the same thing many times.

When you pump them with the engine off you soon loose all the vacuum in the booster and pedal will get really stiff b/c it loses the vacuum assist.

Consider the possibility that you got a bad master cylinder out of the box. I’d bleed it all again and see what happens.

Other than air & the mc a soft brake pedal can come from deteriorated flexible brake lines. They can balloon under the pressure. You’ll have them at least in the front and maybe in the rear. Yours are something like 12yrs old. Replacing them wouldn’t be bad even if they don’t have anything to do with it.

They are calling the part a dump valve.

To add to Cig’s excellent post, the way to test a brake booster to see if it’s working is to pump the brakes with the engine off, depleteing the vacuum in the booster’s front chamber, then with the pedal depress turn the engine on. The booster will make the pedal go soft if it’s working properly.

I agree with changing the flexable lines at the wheels. But I’m also wondering if there’s still air in the system. How are you bleeding the brakes? Are you putting the plastic tubes from the bleeders into small plastic bottles with fluid in them, so that any “drawback” pulls fluid into the cylinders and not air?