Soft brake pedal

I just picked up a 1998 K2500 and the brake pedal is soft and able to go all the way to floor. I have inspected all the brakes and they have plenty of
life left, I also bled the lines at each wheel. Is there anything else to check before replacing the master cylinder?
thanks mike
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Leaks Fluid Level Fluid Condition Rotor Thickness/drum diameter
http://users.eastlink.ca/~smackie/brakedaig.pdf
Steve
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Steve Mackie wrote:

I would be curious as to what Rotor thickness,drum diameter would have to do with a soft brake pedal?
Ian
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Not so much the soft brake pedal, the OP stated "brake pedal is soft and able to go all the way to floor." Just one of those things that takes 2 seconds to check, so why not.
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Steve Mackie wrote:

I would agree that it doesn't take long to check those items, but there is no relationship between those measurements and the problems that the OP is experiencing.
Ian
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mike wrote:

I would make sure your rear brake shoes are adjusted properly. You could try changing the brake fluid, but if neither of the above help....yes, a master cylinder change may be in order. This year of GM truck was notorious for lousy feeling brakes. GM even came up with a brake pedal rubber pad that was quite a bit thicker then the original as part of their "solution" to the soft brake pedal and "low" brake pedal problem. In most cases, adjusting the rear brakes properly, and making sure that the front rotors have an excellent surface finish (if the brakes have been done recently, a poor machining job on the front rotors can cause problems) will make the brakes perform as good as they can. In some cases, I've found that replacing the master cylinder has done wonders. But again, this year of GM truck (and many other years) simply sucked when it came to brake performance.
Ian
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shiden_kai wrote:

    Tell me about it. I just did brake repairs on 2 C-2500 6 lug (7200 GRVW) pick ups. Both are Ex-Fleet vehicles with over 200,000 on them. Both have Extreamly soft brake pedels. They get hard when bleeding the system with the engine off. Turn the engine on and pump the brakes they feel like mush. One of the trucks has new rear shoes and wheel cylinder. The other has no front pads and calipers. Both have all new lines running to the rear end flex hose.
    The one with new front brakes will hold the truck still untill the rear tires overcome the brakes and spin/burn rubber on concreate. Yet take the truck down the driveway, and you don't want to even go 20 MPH.
    Other then master cylinders on these, and rear brake adjustments, any other suggestions? Charles
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Charles Bendig wrote:

Something to ponder... Since I've owned my truck (1999) it's had less than ideal pedal feel. Replaced 1 wheel cylinder, several sets of pads over the 135,000+ miles I've owned it, master cylinder, had it professionally bled, still had crappy pedal feel. I had to replace my brake booster on this 95 K1500 5.7L about 3 months ago due to it pissing air in the cab. This helped the power assist but I still had a weak pedal. 7 days ago this rebuilt booster started pissing air so I had to replace it also. Nice Murray's products! This time however, I left the master cylinder attached to the brake lines & let it just stay in this suspended position while changing out the booster (I did support it somewhat). As it normally sits in the truck, the master cylinder tilts back towards the driver. The suspended way it sat while I did the swap job caused the master to tilt quite a bit forward. I did no bleeding after this fix. The brakes have been in operation a week now & the pedal travels much less and is unbelievably quick to get VERY hard. Operation is FAR better than ever. I'm thinking that maybe some air sat in the end of the master cylinder & would not bleed out no matter how many times it was bled. Tilting the master forwards for awhile might have moved some air enough so that normal operation self bled the master. No matter what, I'm extremely happy now. Similar thing happened to an Austin Marina I once had. The brake lines came off the master in an upward spiral & then down to the proportional valve. Stupid design to in my eyes. I could not get those things bled until I cut the lines at their highest point & installed some petcock bleeders - instant fix. Sometimes on a motorcycle you have to crack open the top banjo bolt in the brake & clutch systems to make them bleed all air too.
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Last summer my neighbor had a 1 ton ford and couldn't get a brake on it. He had replaced every pad/shoe, master and booster still no brake. I was over doing a little inspecting and had a set of wheels off one side on the rear. Just happened to be looking when he pressed the pedal and the drum was actually stretching out when he was braking. Pulled a drum and it was wore out. A local shop had actually turned the drum a few months back and even though they were well out of spec the shop either didn't bother to measure them or didn't care. Even though the front brakes do a larger amount of the braking not having rear brakes really makes a diffrence. Also your front rotors will heat up and allow brake fade much quicker if they are too thin. A master cylinder usually tests easy. The most common problem is bypassing. Test this by holding constant pressure on your pedal with the engine off and make sure that the pedal does not slowly go down. If you don't want to simply throw away $ rebuild your own master cyl. They are extremely simple to rebuild. You can buy the rubber seals for less than 10 bucks and you probably already have a small hone in your garage. Also a hydraulic guage is a inexpensive tool to have in your brake toolbox. Make sure your proportioning valve is working. Check each wheel end for proper pressure. I would have to guess that 95% of vehicle owners do not change brake fluid. Even though brake fluid doesnt wear it will absorb water. Brake fluid is good at what it does because it allows virtually no compression but add water and it will compress. Fluid in a hydraulic brake system that is water contaminated will put you in a situation where your master cyl is displacing all it can but due to the fluid compressing will not apply ample force at the other end of the line. Also water and steel brake lines don't mix. You get a little condition called rust. Rust in brake system equals plugged orfices, wear on seals and in worst case scenario a line that ruptures when you stand on the pedal in a emergency. CRASH. Also it will cost you some cash but look at DOT5 brake fluid. It compresses even less than DOT3 and won't absorb water. But there is a pretty good price diffrence and you must make sure that all of you old fluid is out of the system as DOT3 and DOT5 are not compatable. You will probaly have to have a shop clean your system before you switch. If you are doing a complete system overhaul such as on a restore where you are replacing all components you need your head cracked if you fill with DOT3.

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Part 2 Pads and shoes are made of diffrent materials. They have a rating system on how aggressive they are. Some materials will last a hell of a long time but don't have the bite when it comes to stopping. Your softer compounds generally grab better but youll be changing them every 20K. Then you have some of your high end brakes, ceramic and carbon composites that offer both wear and hold. My wife drives a 04 Pontiac minivan. The original pads lasted about 25K and I replaced them with a ceramic pad. They were inspected a week ago with about 20K on them and looked like they were worth at least that much more. And theyll put you through the dash.

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INLINE

Correct
Brake fluid is

Wrong
Fluid in a hydraulic brake system that is water

Wrong again. Water is no more compressible than most brake fluids. Water has a low boiling point and in a panic stop can boil and become a gas, which is compressible.
Also water and steel brake lines don't mix. You

Correct
Also it will cost you some

Wrong... Dot 5 is silicone based and it's molecules are farther apart than Dot3-4 which makes it somewhat more compressible.
and

Correct, but that isn't necessarily a good thing. Moisture gets into all brake systems, no way to stop it. Since Dot 5 won't absorb it, the water will pool in the bottom of the lines and other low points and corrosion becomes more of a problem than with the glycol based fluids. Also, pooled water becomes a serious problem in freezing weather.
But there is a pretty good price diffrence and you must

Correct
You will probaly have to have a shop clean your system

Dot 5 can damage seals and other soft parts if put into a system not designed for it. Unless he is planning to run his truck at the local circle track, he should stick with Dot 3( if he is, he could go to Dot 4 which has a higher boiling point and IS compatible with his system).
Dave
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Stephen Young wrote:

    On cars and trucks that have sat with broken brake lines. First step in bleeding is Gravity Bleeding. This means you take the cap off the master cylinder. Open the bleeder screws to the point that fluid starts to flow freely. Sometimes you have to remove the bleeder screw for a few minutes. I use a catch pan and let each wheel gravity bleed for 5 to 10 mimutes. Keeping the master cylinder from going dry by topping off, this get the old brake fluid out of the lines, removes air from new wheel cylinders, calipers and the master cylinder. Then I bleed them from there.
    You can bleed lines by first cracking them lose. Then having some one pump up the brakes and slowly opening them and closing them. Charles
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