Brakes pedal soft for most ot its travel.

[BigRedWeather]

About a month ago my brake pedal was soft until the last 20% of it travel. For that last 20% of travel it braked well and during the first 80% of travel is braked the truck slightly. If I turn the truck on and park it, I can pump the brake a few times and the pedal gets firm after 40% of travel, but then when I drive again, it only really stops the truck for the last 20% of travel.

I found one rear shoe had lost the brake material, resurfaced the drum and but new shoes on both sides. I thought this would explain the problem - because the shoe would expand to contact the drum "taking up most of the travel" and then the other brakes would get pressure and stop the truck. BUT, after the new shoes I still have the same problem.

I have removed the ABS from the equation by unplugging the brake fluid reservoir indicator sensor - as my factory manual told me to do - but that does not help.

Any ideas what would cause this sudden change? The brakes were firm close to the top of travel for a few years and then all a sudden one day they got soft.

 

[79jasper]

I'm currently diagnosing the same thing on mine. I'm somewhat sure my booster is good. I just replaced the master cylinder because I could feel the pressure bleed off while holding peddle down. I need to bleed the rabs valve and flush the brake fluid. Earlier my rear tires actually locked up while braking(back end kicked over, luckily I wasn't turning yet). While driving I have to pump them. Usually second pump and they "work" but they try to grab. Mind you this truck sat for a year. Before that I drove it for a few months, and before that it had sat for a long time. I think my grabiness is the brakes practically haven't been used in forever. Also mine try to stay on after letting off, slighty but you can fell it. Which I believe is from needing to grease the sliders. I've also replaced the drivers rear wheel cylinder( I know I know, should've done both). The last bit of pedal travel is the rear brakes.

Anyways back to you bud, does your ABS light work at all?
Are you sure your booster is good?
Can you have someone pump the brakes while you look at the rubber hoses to look for expansion/swelling?

 

[BigRigTech]

Master cylinder might be junk, I've seen this before. If all your brakes are adjusted up, no leaks and no air at the wheel ends then the M/C could be bypassing internally. I had this happen to me on my 85 F150....I had to pump it up a couple times before I was going to stop to get decent pressure in the pedal. New M/C fixed it.

 

[79jasper]

Also, I read that we shouldn't pump these brakes. Even when bleeding. Just push down, open bleeder, close bleeder. Do that until it feels hard.

 

[BigRedWeather]

I'm currently diagnosing the same thing on mine. I'm somewhat sure my booster is good. I just replaced the master cylinder because I could feel the pressure bleed off while holding peddle down. I need to bleed the rabs valve and flush the brake fluid. Earlier my rear tires actually locked up while braking(back end kicked over, luckily I wasn't turning yet). While driving I have to pump them. Usually second pump and they "work" but they try to grab. Mind you this truck sat for a year. Before that I drove it for a few months, and before that it had sat for a long time. I think my grabiness is the brakes practically haven't been used in forever. Also mine try to stay on after letting off, slighty but you can fell it. Which I believe is from needing to grease the sliders. I've also replaced the drivers rear wheel cylinder( I know I know, should've done both). The last bit of pedal travel is the rear brakes.

Anyways back to you bud, does your ABS light work at all? I HAVE DISCONNECTED THE BRAKE FLUID LEVEL INDICATOR CONNECTOR FROM THE MASTER CYLINDER, TO TURN THE RABS OFF - ACCORDING TO MY FACTORY MANUAL AND AS A RESULT MY ABS LIGHT IS ALWAYS ON NOW. I HAVE ALSO REMVOED THE SPRING FROM THE ABS VALVE.
Are you sure your booster is good? IN THE LAST THREE YEARS I HAVE REPLACE THE MASTER CYLINDER, BOOSTER, BOTH FRONT CALIPERS, PADS, SOME BRAKE LINES, A DRUM, RESURFACED BOTH DRUMS, BOTH SHOES, TWO REAR CYLINDERS, SOME OR ALL OF THE FLEXIBLE LINES SO I ASSUME THE BOOSER AND MUCH OF THE SYSTEM IS GOOD.
Can you have someone pump the brakes while you look at the rubber hoses to look for expansion/swelling?

THATS A GOOD IDEA, I WILL TRY IT

I am going to investigate how to remove the RABS (effectively, I may have already done this, see above) and the rear wheel speed sensor at the rear differential so I have just the plain vanilla brake system. I would like to find someone who has done this and hear the results.

Thanks for your post

 

[79jasper]

That rear speed sensor is the VSS. It runs more than the rabs, it also runs the speedometer. And plays a big role in shifting automatics(which I see you don't have). Supposedly the rabs is disabled anytime the abs light is on. That's why I asked if your light works. You can retrieve the codes by yourself.
But yes you can take it out and replace it with a piece of brake line. Or you can take the rabs valve apart and remove the spring inside.
Also I'm ALWAYS back heavy, so I can't afford to chance taking it out. And slide sideways from a straight line like last night. I would rather have it working.
on edit: I missed where you answered my questions. So yes you have disabled it. Does the peddle still feel crappy?
Also how old and what color is the fluid?

 

[Double-S-Diesel]

one side shoes wore out check park brake cable, make sure the arem is returning, I bet its not

 

[79jasper]

one side shoes wore out check park brake cable, make sure the arem is returning, I bet its not

In his other thread he said one side locks up, couldn't that just be improperly adjusted shoes? Or a bad rubber hose?
I know for a fact the rabs system itself will not cause a wheel to lockup, that's what it prevents.

 

[79jasper]

Also not to hijack, but does anyone know which wire you ground to flash out the abs codes?

 

[Maverick1701]

maybe something vacuum pump related?

 

[madpogue]

Vacuum pump cannot possibly create a soft pedal. If it fails or acts up, you can get a hard pedal.

 

[Maverick1701]

Vacuum pump cannot possibly create a soft pedal. If it fails or acts up, you can get a hard pedal.

lol well that was a "swing and a miss" on my part... I knew they either got really hard or really soft when you loose vacuum.

 

[79jasper]

lol well that was a "swing and a miss" on my part... I knew they either got really hard or really soft when you loose vacuum.

I heard some older gm models would lock up the brakes when the booster failed. Maybe the design thought was it'll apply the brakes when it failed so you wouldn't keep driving with no brakes.
From what I've seen and read, a vacuum related problem can be hit and miss.

 

[BigRedWeather]

on edit: I missed where you answered my questions. So yes you have disabled it. Does the peddle still feel crappy?
Also how old and what color is the fluid?

Yes, it is disabled, but the peddle is the same, very soft the first 80% of its travel.
My fluid is about 2 years old and slightly darker than clear.

 

[BigRedWeather]

Also not to hijack, but does anyone know which wire you ground to flash out the abs codes?

There is a connector under the glove compartment. If that is disconnected one end of the wire can be used to make it give ABS codes. When my ABS was functional, it would give me the all ok 16 flashes.

Other thoughts... When I have adjusted my rear brakes in the past it would reduce the amount of rear wheel lockup, but not stop it. I have also really cleaned all parts inside the drum brakes with a wirewheel, and lightly greased where the shoe contacts the back plate. I took this truck to a very good brake shop in town (300,000 people) and they told me to take it to the stealer. I'm thinking of replacing everything in the brake system, maybe starting with the parts I haven't replace recently. I'm still studying it.