No brakes

[grovey]

I lost the brakes on my  250K  02 Sierra yesterday.  The ABS  braking has been acting up the last 4-5 days  during  say... 1 out of  6 normal stops. Doing the typical GM moaning  and groaning it does if it's slippy out  or the rd is  very bumpy at a stop sign BS.  Doing that under normal conditions though.  I couldn't make it do this when I tried several times, so not sure why that is.  My reservoir is  1/4 " low of fill line, and I don't suspect  that's related to this  there's  no seeps or leaks I can see under the hood or under the truck anywhere.  I pulled the ABS fuse and the only difference was it made the ABS and brake lights come on. When replaced they went out. I replaced the ABS module with a used (140K ) unit 10 months ago chasing another issue.  They're so weak I just ran over the top of my car ramps because they wouldn't hold even creeping.   Anyone had these issues? 

[davester]

It's unlikely to be the abs module, as it's designed to not significantly affect braking, even if it fails.

 

More likely, given that you say there is no leaks and the brake fluid isn't going down, that your master cylinder has failed.

 

I would also suggest examining the brake lines, as depending on where you live, the lines are known for rusting, particularly the 4 lines running along the frame from the driver's side front wheel to the abs controller, and also the rear line running along the frame above the fuel tank (hard to see without an inspection camera).

[riverbanks]

The moaning is abs activation,one or both front wheel sensors is dirty,get the truck on the proper scanner and see where the fault is

[grovey]

No leaks seen under hood or truck on drivers side frame rail . I had a  hard line replaced there last yr along with all the bleeders and a pass side ABS sensor.  

[grovey]

With the truck running...  The pedal goes to floor feeling like mush the last inch.  With truck off it feels like it "pumps up" a bit with more resistance, but still goes to the floor.  I started the truck after pumping them this way, and they still won't hold the truck in gear past idle. 

[riverbanks]

That last inch mush,is you pushing past the weak seals in the master cylinder,time for a new one

[grovey]

Well I was wrong. It was a leak! A line burst on the outside of driver frame rail.  My buddy tried to patch it with a compression fitting, and had to pry it from the clip on the frame to do it. Well... once together that line burst again where the factory bend goes over the rail. Perhaps prying it out made another weak spot .  Fixed with a new piece of line from ABS unit to union of existing line. Now I can't get the front brakes to bleed. Just a bubble, or dribble. Been around the truck  6 ? times, and still nothing.  Parked it on a steep incline looking for some help from gravity, but no dice. 

From what I've gathered it could be the ABS unit that now needs bled to get the air outta it.  A special scanner is used to remove air from the unit into the lines so it can be bled what I've seen, and rebleed, repeat.  But , I saw a video of a guy breaking the lines loose on the unit itself to bleed.   Anyone been there?

[evad]

this is kinda farm boy tech but don't mess with the abs unit,,fill up the master cylinder crack all the bleeders and let it sit overnight then try again ..if no luck go get a new master cylinder

[davester]

The abs controller can hold a small amount of air in it, which is what more advanced scanners/computers enable you to bleed out, but it won't alllow air to be drawn into the lines, other than the fittings not being clean and properly tightened to it.

 

Overall the bleeding process is:

a) bleed all 4 corners, starting with the wheel furthest from the MC to the wheel closed to the MC

b) perform the abs controller purge procedure (requires a more advanced scanner/computer to do this)

c) bleed all 4 corners again, in the same order.

 

At the end of a), all 4 corners should have shown no air in the lines.  If they continue to have air bubbles, something is wrong, from lines not being connected/tightened properly, to using a vacuum bleeder having too much vacuum and drawing air through the threads of the bleeder screw.

 

And some people will do a "redneck" method to get the air out of the abs system, which is to (after doing step A) go to a dirt road and then do multiple panic stops to activate the abs system each time.