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Brakes? What brakes... pfff grrrr


wishybone2

Question

2004 GMC Sierra Crew Cab z-71

Dual piston calipers Rear

No traction control

Delphi ABS with out bleeders

 

 

Soooo... I changed the the rear calipers on my "yard" "plow" truck... thats imporant for later....

 

That was a couple months back... Brakes got a little spongy after but NBD

 

So then I blew two brake lines and fixed them... Bled and they came back to OK

so I figured I would change my front calipers as well because the bleeders were stuck and I figured I wasnt getting a good release...

 

After changing the front calipers... NO Pressure... tried bleeding many times... well turns out the ABS module only can be bled with a tool...

 

Problem is my ABS motor is shorted... dont want to fix because its just a truck we use to plow the farm driveway... and thats big $$

 

So I say... Well I have the time I can get it to bleed.... YA RIGHT! After hours of watching air bubbles go by (open bleeder, pushdown, close, up slowly, open bleeder, push down)... I mean at least 10 hours and bleeding and tapping on everything... well I had a good pedal when the truck wasnt running but soon as it was started... right to the floor... everytime

 

So I said to myself... Well I can bypass all that stuff and that will get rid of the air in that pesky controller... well yep... brakes bleed great when the trucks not running... no air in the lines... but soon as i start it... bang right to the floor... OK is there somthing I need about this thing or should I just drive into the old farm pond out back....????

 

Thanks for your time in advance... I understand that this is kind of a modification which is a no no here but it wasnt intended to be and and the modfication did not cause the issue... it was there the whole time...

 

Also please dont lecture me on how great ABS is... I know its great for some applications... just not plowing my 10 acre yard... I just want to have brakes again...

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So I got a new Master Cylinder... bled it and put it on... got a little better after a while... bled some more air out of the lines... but still no enough brake pressure to hold the axels from spinning when I put it R.... any ideas guys?

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So you bypass abs controller, bled brakes, and pedal still drops? Sounds like somthing else could be going on? Trouble with master cylinder?

 

 

Yes only drops when running... i thought the same thing.. is there some type of safety in the master cylinder that I need to reset... or the small junction box under the MC... I dunno what it does but it has a sensor on it...

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Are you 110% sure there's no leaks? Just asking because I've never had a problem bleeding an ABS system without a scan tool. I just did it with a vacuum bleeder, then followed up the traditional pump, hold, / open, close way. But if it ends up being that the ABS unit has an air pocket, I'd just flare up some new lines & couple them to the old ones & be done with it.

 

You won't get any preaching from me. If I had my way I'd gut my truck of anything that relies on transistors that doesn't control convenience features. IMHO electronics have no place under the hood.

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Are you 110% sure there's no leaks? Just asking because I've never had a problem bleeding an ABS system without a scan tool. I just did it with a vacuum bleeder, then followed up the traditional pump, hold, / open, close way. But if it ends up being that the ABS unit has an air pocket, I'd just flare up some new lines & couple them to the old ones & be done with it. You won't get any preaching from me. If I had my way I'd gut my truck of anything that relies on transistors that doesn't control convenience features. IMHO electronics have no place under the hood.

 

 

Im sure theres no leaks... but now Im thinking I could have got air in my master cylinder in the beginning and that could be my whole problem... boy would that be dissapointing...

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Yeah got to bench bleed them first .... otherwise you'll end up bald/grey a couple decades sooner than normal, if you know what I mean.

 

I use that as my excuse .... :)

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Sorry it's late, but if you find some small nails with a head on them, install them where the lines connect to the M/Cyl, you can isolate your problem. The pedal should be hard as a rock, with the nails in there. If not the master is bad, or is airbound. Take out one of the nails, see how the pedal feels. You can isolate the problem, front or rear with this method. Don't over tighten with the nails in or you can damage the flare on the lines.

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