after changing brakes all around and the master cylinder bleed lines several times but the pedal is still soft any suggestions on what could be the problem
There is air in the system some where, you can't let the master cylinder run dry or you will need to bleed it separately. If you are sure the break lines have no air and the master cylinder did not run dry, it is possible the master cylinder is bad. Did you have this problem before working on the breaks?
Bleed the lines furthest from the master cylinder first. Example Right Rear, Left Rear, Right Front, Left Front. The master cylinder may need to be bled also.
The master cylinder was bad so it was also replaced and I bench bled it before putting it back on the truck. There is no air coming out of the lines when bleeding them and the master cylinder hasnt ran dry. Could the brake booster or abs sytem be the problem?
It is possible the ABS unit has air in it, not sure if you have to go to a dealership to get them to put it on their machine and open up the ports or not. You might check with the dealer or mechanic.
Looking for a Used Sierra 1500 in your area?
CarGurus has 59,972 nationwide Sierra 1500 listings starting at $1,299.
Search GMC Sierra 1500 Questions
Are you a UK consumer? CarGurus now has a discussion forum in the UK.
GMC Sierra 1500 Experts