Gave up on the pressure bleeding, didn't help when we were bleeding air from the ABS module. Spent 2/3 hours gravity bleeding the rear brakes because of the air in the ABS module. Even when using the 'bleed valve' on the module.
Big lesson learnt was that we should have replaced all the pipe from front to rear.

 

Rule #1 with 1998 and newer: DON'T pump the pedal to bleed, (old school technology doesn't work on this system). All it does is make the situation worse.

Remove the master cylinder cover, fill the reservoir, leave the cover off and gravity bleed one wheel at a time. It doesn't matter where you start, or what order you do them, but they ALL must be done. Once a steady stream of fluid exits the bleeder, with no air bubbles, shut the bleeder and move on to the next wheel. Do not allow the reservoir to run dry on fluid during the gravity bleed. If the red brake warning light, (in the instrument cluster) is illuminated with the ignition in the RUN position after all 4 wheels have been bled, give a quick, hard, jab & release to the brake pedal to center the combination valve. If the valve is stuck, you might have to repeat the "jab & release" a couple of times. FYI: power/pressure bleeding is NOT recommended on this system. It can, and most likely will, damage the ABS unit.

If this bleeding method is done correctly, the auto bleed is rarely necessary. As mentioned earlier, the auto bleed forces any trapped air in the ABS unit into the steel lines. Gravity bleed is again necessary to move the air out of the system at each wheel.

 

Captain Hook,

Following this thread has got me confused.

I've looked up the bleeding procedure in my GM manual and it outlines a method that consists of pressing on the brake pedal, holding it, and then having an assistant open the bleed valve to let out the extra air and fluid. This is definitely different from the old-school way of pumping the pedal with the bleeder valve open to push fluid through the system that I've used on non-ABS cars.

Is what I described from the GM manual a form of "gravity bleeding"? Or, is gravity bleeding as you described different than the GM manual, simply consisting of opening the bleed valve at the caliper and letting the fluid flow without touching the brake pedal?

I'm attaching scans from my 1999 GM S/T Truck Service Manual that describe bleeding the brake system. Can you look at the pages and tell me what I am missing?

 

Thanks for this info, it would appear that we've been adding to the problem and actually putting air in the system. Unfortunately we discovered another leak in the pipe from front to back and will be replacing the pipe completely giving us a chance to use gravity method.

 

 

LOL, yes. You cannot argue with experience.

So gravity bleeding as simple as I think it is? Just open the bleed valves and let the fluid flow while keeping the reservoir topped off? I'm guessing it is advisable to have a long clear tube attached to the bleed valve so to be able to monitor the color of the fluid and if there are any air bubbles coming out.

 

Thanks for the kind words... greatly appreciated.

Gravity bleeding is as simple as it gets. One wheel at a time, until it comes out clear, (no bubbles) at each bleeder. The front brakes have their own separate lines from the EBCM to the caliper. The rears use a single line from front to rear, (along the left frame rail). At the rear differential housing, it splits to each wheel. Due to this long run of line, it can take 15 or 20 minutes to do the first rear wheel. The second rear wheel usually only takes a couple of minutes. Gravity is a beautiful thing, when it comes to bleeding brakes. After all wheels are done, the only thing necessary is hold firm pressure on the pedal, again, don't pump the pedal, and quickly open and close each bleeder, one at a time. It must be done quickly to avoid triggering the centering valve. "Most" of the time, you won't see any air expelled from a bleeder valve.

This procedure makes short work of replacing calipers too. After gravity bleeding, and all bleeders are closed, slowly apply and release pressure to the pedal a few times to extend the caliper pistons, (don't go all the way down to the floor). Then have an assistant hold firm pressure on the pedal while you quickly open and close the bleeder. If you've never done it this way, you'll be amazed how quick and simple it is!

Note: For the gravity bleed procedure to work, the red "BRAKE" idiot light in the instrument cluster must be OFF when the ignition is in the RUN position.The ignition does not need to be in the RUN position to do the gravity bleed. Centering the combination valve can be a challenge at times

 

Thanks for the help. That is interesting about the centering-valve. I always appreciate knowing why I should do one thing or another.

 

Due to ill health had to put the brake problem on the back burner untill a couple of weeks ago.

Finally decided to buy a scanner, the Autel Maxicheck pro. At 170€ it was a good deal and it does the job!!
Just need a couple of hours to do the final bleed and should be back on the road.
Thanks to all for your input.

 

Well, the scanner works fine. no DTCs and the autobleed is good - however still unable to get fluid to come thought to rear brakes. As stated before I have tried pressure bleed and now have even gone for vacuum but all I get is air. The fluid level in the master cylinder doesn't go down at all. Could there be a fault with the master cylinder? I'm reluctant to take it apart as it would appear that there are no service kits available anywhere, unless anyone knows better!