98 GMC Sierra soft brake pedal problem


Superman524
04-11-2006, 12:58 PM
OK guys I need help on this one. 98 GMC C1500 Sierra 5.7 vortec/20" wheels. After 2 master cly. a new vacuum booster, new rotors, new front dual piston calipers, a rear brake adjustment and bleeding the system with the battery disconnected I still have a soft brake pedal. Also if I press the brake pedal hard enough it almost goes to the floor. Is there a proceedure beyond disconnecting the battery to bleed air from abs module? Do I even have a problem with the abs module or is it something else? I'm not sure what to do next but I wouldn't tow anything at this point. I appreciate any and all input. Thanks, Dave

2000CAYukon
04-11-2006, 01:43 PM
I would look at the following:

- Rod adjustment between brake pedal and booster (i.e. how far does the pedal travel before it even starts to push on the booster)
- Bad rubber brake line(s). (i.e. rubber is expanding under load).

Also, if you pump the pedal does it get firm or does it always go to the floor?

Did you adjust the rears or did you have someone adjust them? You want some drag when you spin the wheel (rear end on jack stands). How does the parking brake feel? If it goes to the floor, the rears are out of adjustment or the cable is out adjustment.

I have found that the self adjusting rear brakes need to be adjusted by hand at every tire rotation; otherwise, you can get a soft pedal.

//2000CAYukon

Superman524
04-11-2006, 04:19 PM
Thanks for the reply 2000CAYukon, the pedal has normal travel about 1 1/2" before it pushes on the booster. I forgot to mention I replaced all 3 rubber brake lines. Pedal will continue to go down only if I apply constant hard pressure but never reaches the floor and yes it does become firm when pumped. I adjusted the rears myself and probably gave them just a little more drag than normal the emergency works good. I haven't replaced the rear wheel cylinders there isn't any sign of leaks but the dust caps don't look real good. Also the rear shoes don't seem to be worn a lot... could I have a bad proportioning valve and would that cause a soft pedal? Thanks for your input. Dave

2000CAYukon
04-11-2006, 04:59 PM
You could have a frozen rear wheel cylinder that is not allowing the brakes to be applied in the rear and that can cause a soft pedal. Jack up the rear and have a help press the brake while you turn each wheel by hand. Verify that the rear brakes are grabbing well.

When you bleed the brakes, you should have done RR, LR, RF and then LF.

On the front, the caliper pins should be lubed with brake caliper lube. If a caliper is sticking it can also cause a soft pedal.

Last thing is that you should double check the rear adjustment after it has been driven. If you spin the wheel and there is no drag, you should adjust them again.

//2000CAYukon

Mr. Smith
04-12-2006, 09:42 AM
Don't use reman master cylinders. Buy new from AZ or Advance and bleed. You should have a really decent pedal after you do it.

OK guys I need help on this one. 98 GMC C1500 Sierra 5.7 vortec/20" wheels. After 2 master cly. a new vacume booster, new rotors, new front dual piston calipers, a rear brake adjustment and bleeding the system with the battery disconnected I still have a soft brake pedal. Also if I press the brake pedal hard enough it almost goes to the floor. Is there a proceedure beyond disconnecting the battery to bleed air from abs module? Do I even have a problem with the abs module or is it something else? I'm not sure what to do next but I wouldn't tow anything at this point. I appreciate any and all input. Thanks, Dave

Superman524
04-12-2006, 04:22 PM
I R&R both rear wheel cylinders did new shoes and then bled & bled & bled and couldn't purge air from rear cylinders. Checked and tightened all brake line and hose connections including bleeder valves and abs module lines. Bled rear and front afterward, no air at front calipers but still air at rear. The pedal got worse it now almost reaches the floor and dosen't firm up when pumped. I will follow up on Mr. Smith's advice and focus in on the master cylinder beyond that I have no clue where to look next. Thanks guys.

2000CAYukon
04-12-2006, 06:10 PM
I R&R both rear wheel cylinders did new shoes and then bled & bled & bled and couldn't purge air from rear cylinders. Checked and tightened all brake line and hose connections including bleeder valves and abs module lines. Bled rear and front afterward, no air at front calipers but still air at rear. The pedal got worse it now almost reaches the floor and dosen't firm up when pumped. I will follow up on Mr. Smith's advice and focus in on the master cylinder beyond that I have no clue where to look next. Thanks guys.

OK. Whenever you have lots of air in the line due to a hose being replaced (or wheel cylinder), this is the bleeding process you should use until you start seeing some fluid.

Starting at RR, open bleeder screw, have helper push AND HOLD the pedal to the floor, close the bleeder, have helper release the brake pedal. Repeat until you start to see fluid come out.

Then move to LR. Once you have fluid coming out, you can pump 3 times and crack the bleeder.

Note that since there is a bunch of air in the rear lines, you may need to bleed the LR (using the above proceedure) before you start getting fluid out the RR. This is OK, be patient. Once you start getting fluid out, finish RR then LR.

//2000CAYukon

Superman524
04-12-2006, 07:00 PM
Thanks 2000CA Yukon, I do have fluid coming out both L&R rears but with some air mostly in the LR. I have someone pump 3 times and hold before I crack the bleeder mostly fluid comes out but always with a blip of air more at LR than RR. I bled rears with both battery disconnected and connected. When the engine is running the LR will intermittently have no fluid come out when bleeder is cracked and pedal is pressed to the floor...yes the master cylinder is full, this only happens when engine is running. It seems like it's sucking air in from somewhere...not sure maybe from master cylinder reservoir. Dave

2000CAYukon
04-12-2006, 08:12 PM
Thanks 2000CA Yukon, I do have fluid coming out both L&R rears but with some air mostly in the LR. I have someone pump 3 times and hold before I crack the bleeder mostly fluid comes out but always with a blip of air more at LR than RR. I bled rears with both battery disconnected and connected. When the engine is running the LR will intermittently have no fluid come out when bleeder is cracked and pedal is pressed to the floor...yes the master cylinder is full, this only happens when engine is running. It seems like it's sucking air in from somewhere...not sure maybe from master cylinder reservoir. Dave

Does your 98 have one rear brake line that tees at the diff or do you have separate brakes lines all the way back for the rears? The line (or lines) for the rear are long so it can take some time to bleed properly.

Make sure your helper does not pump to fast. Also make sure that when you crack the bleeder, that the helper maintains even pressure as the brake pedal drops. They should also not release their foot on the brake until you give them the command to do so.

You could try my method to see if you can get all the air out of the left rear.

Good luck.

//2000CAYukon

Mr. Smith
04-13-2006, 09:32 AM
Thanks 2000CA Yukon, I do have fluid coming out both L&R rears but with some air mostly in the LR. I have someone pump 3 times and hold before I crack the bleeder mostly fluid comes out but always with a blip of air more at LR than RR. I bled rears with both battery disconnected and connected. When the engine is running the LR will intermittently have no fluid come out when bleeder is cracked and pedal is pressed to the floor...yes the master cylinder is full, this only happens when engine is running. It seems like it's sucking air in from somewhere...not sure maybe from master cylinder reservoir. Dave

Since you ignored my last post this will be the last help you get. Since you aren't getting any fluid that means you shuttle valve has been set. Your brake light is on in the dash. It cuts off fliud think there is a leak.

I know what to do...You figure it out . Have a great day :rofl:

Superman524
04-13-2006, 10:13 AM
I haven't ignored your last post Mr. Smith, what's described in post#6 was preformed after post#4 and before your post#5. In post#6 & 8 I was responding to post# 4 to what I had already done. My next step was to check master cylinder and bleed according to post#7's method. By the way my brake light on the dash is not on. You have a great day.

MyTOY
04-13-2006, 11:13 AM
I've seen it suggested to raise the end being bled, gently tapping
caliper to remove bubbles, and using line locks to pinpoint the
wheel causing soft pedal. I had alot of work done to my brakes
and since have a softer pedal. I'm going to look for a place that
can flush the system with the equipment to cycle the ABS. My
fluid is looking a little crappy anyway. I had removed one of the
brake lines to replace shock before I learned how sensitive the
fluid system is and I should have capped.

Superman524
04-13-2006, 03:21 PM
Thanks for that advice My Toy I think I'm headed in the same direction. I just finished installing a brand new master cyl. bench bled it according to instructions before bolting it in. Bled entire system as per post#7 no air at front calipers air at rears but mostly fluid. No difference in brake pedal still soft and goes long. Does any one know how to bleed the abs module? Dave

Mr. Smith
04-13-2006, 10:48 PM
I haven't ignored your last post Mr. Smith, what's described in post#6 was preformed after post#4 and before your post#5. In post#6 & 8 I was responding to post# 4 to what I had already done. My next step was to check master cylinder and bleed according to post#7's method. By the way my brake light on the dash is not on. You have a great day.

I sure will...I'm enjoying this thread too :rofl:

MyTOY
04-14-2006, 12:46 PM
I think an ABS scan tool and/or other equipment is needed
to cycle ABS.
Also, I've seen it recommended that the pin in the
metering portion of the combination valve needs
to be held open while bleeding. Good Luck

Superman524
04-14-2006, 04:04 PM
I think your right My Toy go to this link: http://www.findarticles.com/p/articles/mi_qa3828/is_199806/ai_n8787970:

Superman524
04-19-2006, 08:28 AM
I did some research on how to bleed the abs unit. So for those of you that would like to know how to bleed the abs unit, more specificaly the BPMV (brake pressure modulator valve) with or without the tech 1 scan tool go to this link: http://www.syty.org/archives/syty/9605/msg00597.html It worked for me my brake pedal became 75% more frim and dosen't travel to the floor anymore. It may not be as frim as I would like it but I can drive it again without worries. Thanks to all that responded, Dave

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