newman9474

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I have a 1997 chevy 1500 z71. Purchased it with 110,000 miles and now has 198,000. It has always had soft brakes. I've changed the booster, master cylinder twice, fully bled them several times. Had it to 3 different mechanics. Put through an auto bleader. New pads new rotors. Nothing has fixed them. They still feel the same. Mechanics have no idea. Any suggestions?

97RedBull

soft as in how? when the trucks off or on?
my 97 chevy pickup has the same problem, when the truck is off it has good siffness in the pedal all the way at the top but when the truck is on the pedal will go a good of the way down and when im stopping it doesnt feel like the stopping power is really there. i have replaced the calipers pads and rotors and rear brake shoes n hardware and bled it several times but i think i have figured out the problem the master cylinder is leaking internally back into the resevoir but u said u had replaced yours so im clueless.

cams 98 silverado

to answer your problem, we need to break down what brakes are. its fluid dynamics. you start at the pedal. you depress it, that pushes a rod that goes to the booster, the booster is a big diaphram that gets its power from vacuum and increases the pressure at which you are applying to your breaks. and then that rod pushes another piston in the master cylinder, that pushes fluid through the lines. those lines are steel and then to the front of the truck, it transforms to rubber lines and then the caliper. on the rear of the truck it also turns to rubber lines and then enters the drum assembly. our trucks, with the vacuum brake booster, puts out around 450 psi of brake power. so that pressure travels through the lines and thats fine because these lines are steel. but then, they enter rubber lines by the caliper. rubber has its tendency to stretch some when theres pressure inside it. like a tire. so then at 450 psi, your rubber parts of the line, stretch and balloon slightly. thus, you are increasing the amount of fluid in that line at that one point and that fluid and pressure is not fully applied to the caliper. that right there is definition of spongy brakes.

on a note, if your mechanic did not suggest this alternative which is to change out the brake lines going to the calipers to stainless steel lines, then I would want to hit him. This is the first thing People would look to after bleeding the brakes. That rubber gets old, and likes to stretch more. That is why I bought a hydroboost system and ditching the vacuum. I am switching out my rubber lines for stainless, the kit that has all the lines is like 60 bucks. the hydroboost system runs off the power steering pump, and builds pressure from that, not vacuum. So, I will have in upwards of 850+ psi of fluid pressure flowing through my brake lines. those stainless steel lines will take out the spongy in my pedal even more.

The reason your brakes are hard when the truck is off, is because you dont have vacuum in the booster from the motor, and that hardness, is purely master cylinder. that hard feeling is called manual brakes. your manually pushing the valve in the master cylinder and pushing all that fluid through the lines. A good thing to do, to learn much more about brakes, is www.howstuffworks.com look at brake booster, and master cylinder and brakes.

d_slat

when you say soft brakes, do you mean when you're stopped you can push the pedal down quite a bit? if so, but it doesn't go down too far to stop when you drive it, there's nothing wrong. I was an auto mechanic for 5 years at a chevy dealer, and we had a few complaints on this, but it's the way they are.
(if you have to push the pedal down quite a bit to get it to stop, that's a different story though)

scott.robinson

Hey i have a 97 chevy k1500. similar issues. i blew a brake line going around a rotary. someone cut me off and i jacked up on the brakes, my abs kicked in and foot proceded to the floor. i replaced the brake line on the side of the road with the help of my dad and we bled the passenger side caliper,(related to the line that blew). we got the truck home and only had about half of the normal braking power because there was air in the system.. so we got two new calipers because the drivers side bleeder was siezed. i proceded to getting 2 new calipers, pads, rotors. we replaced everything and started to bleed the brakes starting from the right rear to the front left. i managed to get good pedal, then i took it for about a mile drive and noticed my pedal started to get spongy, like air got back in the system, and then i could put my foot to the floor and have very soft, delayed braking...

i then replaced the master cylinder and that didnt make a differance at all. the booster is fine, i have plenty of vacume and there is no fluid in the hose. i also used a tech two scanner to cycle the abs while i was bleeding it after i replaced the master cylinder, which was bench bleed fyi. even after all this the brakes are still soft. the only thing i can think of is the abs module and maybe the dump valve associated with it.. Any thoughts would be greatly appreciated.

skeeter5

When ypou find what is causing this condition, please post the fix. My son-in-law has a 2001 GMC and it has the soft brake petal. I don't think that its the master cylinder.

As to the "rubber" brake lines, if I remember correctly these are rubber covered with steel then rubber again.

Please keep use posted.

My .02.

scott.robinson

after about 1000 dollars the truck is fixed. after i blew the brake line, one thing lead to another and i stumbled upon many things wrong with the truck.

when i stomped on the brakes, the abs kicked on, when it did that the ABS dump valve did not properly seat, and also one of the solenoids stuck open. also my master cyclinder was leaking by. in addition to that my rear cylinders were blown(both of them). the combination of these things and air in the lines lead to a messed up situation where i had no brakes. I went to a junk yard and picked up a new module for 75 bucks.

in my abs module one of the solenoids that dumps brake fluid into the low pressure resevoir was stuck open and it would not allow me to pump up my rear brakes because it blocked them sending fluid to the rear cylinders and instead it dumped the fluid into the resevoir(located in the module not the master cylinder). from there the computer is supposed to cycle the solenoids on and off and uses the electric motor on the module to pulsate your brake. there are 3 solenoids that can dump fluid into the resevoir and if one of the solendoids is stuck open it blocks either your LF,RF or rear brakes and dumps that fluid into the resevoirs. make sense?

next thing, the master cylinder was allowing fluid by because one of the o-rings was bad.... 70 dollars later and a good bench bleed lead to no more brake pedal fading.

we replaced the front calipers, rotors, and pads so we could develope pressure easier in the front brakes to make bleeding easier. aslo we replaced the rear drums, shoes, cylinders, and all the hardware and also replaced some brake line. in doing so we could bleed the brakes easier.

next thing, bleeding brakes is a bitch, we said screw bleeding and sent it over to a shop and had them power bleed it and use a tech 2 scanner to cycle the abs to release all of the air trapped in the module.

end result, powerful brakes that lock up at just over 2/3 pedal. quite amazing for a 6500 pound truck and the abs works like a charm also.
What i have come to realize with my truck, and probly most old chevy trucks is that you cant expect just one thing to be wrong!

Starboy

Hey, Scott. Hilarious note. You're so right about these veteran Chevy trucks. It's not reasonable to just plug in one part and think everything will be fine. It's much better, as you said, to think in terms of systems and work projects. Often it's not necessary to purchase all new parts, but it is necessary to verify that they're all working properly. Brake systems especially, wouldn't you agree?

97GMC

Quick question and Maybe someone has an answer.... * Had an issue where my ABS was kickin in sometimes whenever I stepped on the brake pedal and other times just coming to a stop at the last few inches. And braking while backing up it was even worse. Felt like ABS but it was sometimes one slight thud and other times many but not in a unison timing. Replaced all front hardware from the master cylinder on. It cured 75% of the times this would happen. Checked the rear breaks and they were in bad need of adjustment, cut down the issue another 10%. Here's the thing... Plenty of high pedal strength, Stops without any issue. Only happens now at the last 6 inches of the truck moving before a compete stop and then I get one small Thud and then the pedal decreases towards the floor about 1/2". The pedal btw is at 2/3rds when depressed and stays there until the last few inches... literally only the last few inches, then I get the thud with a 1/2" drop in pedal. Now my mechanic and myself are scratching heads.... all the braking components are working with zero leaks! Any ideas of what this could be? This is on my 97 GMC K1500 5.7L 4x4 PU Thank You in Advance!

greg627

I have 97 K1500 Z71 was having simalar problem tried everything i could think of no joy. finally replaced abs module end of problems. good pedal no pump going off and no drop in pedal just berfore you stop