When you bleed the brakes did you push and hold or did you pump 5 times and hold?
The first I bleed the brakes, I did 5 times. No better! I read the GMC service manual and it said to slowly depress one time and hold. Still the same. I did wait 15 seconds between each bleeding session
I removed most of the fluid to replace the rear brake line. Then I pumped the brakes with the master cylinder filled and cracked the fittings on the master cylinder feeding the front and rear one at a time to release any air from the master cylinder.
Did you bleed the front brakes at the calipers?
yes, three different times along with the rear.
Did you gravity bleed the rear then pedal bleed them?
not really, let the fluid set a short while but fluid did not exit the line
is the violent action in the rear section of the master cylinder during pumping the brakes with the engine running sound normal?
You have one of two problems. either there is still air in the lines or the mastercylinder is bad.
Yes. You will have a a stream of fluid that will shoot up out the back sectiom of the master cylinder.
before I close out, what is your thoughts on that violent action in the rear chamber of the master cylinder?
That is normal. it is from the pressure the master cylinder creats. The pressure will vent when it reaches a certain pressure backinto the master cylinder so it does not blow the lines.
Try bleeding the rear again but this time bleed them 6 times each
If i replace the master cylinder any special venting or bleeding I should do.
You will have to bench bleed the master cylinder before you reconnect the lines to it. You can bolt it up to the booster to do this but use the bench bleeder kit that will come with it first.