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I got this 2002 Chevy K1500, a 5.3L engine, automatic transmission with 4 wheel disc brakes and 135000 miles on it. The truck came in with excessive brake pedal travel going almost to the floor. If I release all the vacuum from the brake booster and hold the brake pedal in hard, pedal travel is barely an inch. As soon as the engine is started, the brake pedal instantly falls to the floor. So, thinking I had a faulty booster I installed a new one with the same results. What am I missing? Rear brake pads are worn down to 3/32" and front is 6/32". Is this new brake vacuum booster defective? Or is my diagnosis wrong? Or am I missing something?

make sure all the caliper slides are working properly. i've found frozen caliper slides as well as calipers frozen on the mount causing low pedal issues. bad front wheel bearings will do this. is the customer complaining of a pedal fade under 5 mph. if this is the issue you may have a bad speed sensor in one of the front hubs. as a rule on many power brake systems (hydro boost being an exception) you're able to push pedal to the floor at a stand still.

I noticed this truck has aluminum brake calipers. And I am wondering if that contributes to a low brake pedal?

alumunim bake calipers shouldn't really have an impact on this unless they're flexing on the slide. i'd also pressure bleed the system if it hasn't already been done. i've had issues with brake pedal height and what i felt was an insignificant amount of air came out and problem solved. i've also had issues where the abs module on the drivers side frame rail was weeping ever so slightly causing an issue. if there is weeping issues at the module i strongly suggest replacing all the lines. we purchase the ones with the braided line in it from the dealer and make the rest.

To help isolate this, you can pinch off the brake hoses one at a time.

Quick question, does the pedal drop as the vehicle is moving and you are trying to slow down? Or does this just happen at stops?

My first guess would be the caliper slides are frozen. Most likely on the back. The reason the pedal gets soft when the engine is running is because the extra assistance probably bends the rotors over against the other pad instead of sliding the caliper.

pinching will kelp but i will warn you that a little too much zealously with vice grips will cause line damage. if all i can lay my hands on is a regular style of grips i'll use two nickels placed against the line and the jaws against the nickels. my first thought about the slides still stands and on rare occasions i've seen bad wheel bearing cause this by leaning into the piston of the caliper.

steven kiser wrote:pinching will kelp but i will warn you that a little too much zealously with vice grips will cause line damage. if all i can lay my hands on is a regular style of grips i'll use two nickels placed against the line and the jaws against the nickels. my first =though about the slides still stands and on rare occasions i've seen bad wheel bearing cause this by leaning into the piston of the caliper.

Excellent advice. I remember customers with diesel engines sitting on the service drive and pushing the pedal to the floor while the vacuum pump built up. Try this on the road and you will be going in circles. Hard to get them to understand but if the pedal is operating properly while driving is a key factor.

protraxrptr17 wrote:My first guess would be the caliper slides are frozen. Most likely on the back. The reason the pedal gets soft when the engine is running is because the extra assistance probably bends the rotors over against the other pad instead of sliding the caliper.

Good idea. So, I pulled both rear wheels and calipers to find out the slides haven't been working (rusted) for many miles. So I replaced both calipers, mounts and new pads.

Same problem - - - pedal goes to the floor. As my former boss would say: Now I'm mad.

now like i stated before the pedal will go to the floor if you push hard enough. while driving the truck and stopping do the brakes engage and stop and then if you push harder the pedal will reach the floor? if it stops fine and under pressure pedal drops the problem may be solved and your forcing the pedal and thinking the problem of a low pedal persists.

GM's are notorious for this. They have sooo much power you can get them to the floor in the bay, but they stop fine with much less effort on the road.

If you're pinching off hoses, I use special purpose tools just for that. Never used vice-grips, ever.

i agree with ric about the special tool. i use one as well, i just mentioned the nickel idea because when "pinch" was mentioned, vice grip popped into mind. i use a vice grip type tool with pads on the jaw end and it will not allow you to close it completely. years ago i would have issues with low pedals being caused by load pressure control valves for the rear brakes but i haven't seen this in years.

In a former post I mentioned that I installed new calipers, brackets and pads. After I went for a test drive it is working fine. So I have to assume that by having the wheels on and vehicle speed in the brake system it makes a difference. Customer picked truck up and so far it is working real well. Pedal only goes in about 1/3 of the way to floor.

Thanks for the help men! It's great to have peers that have ideas, suggestions and input.

thanks for keeping us updated