1998 Chevy Silverado brakes?

[Tim]

I get good pressure at the brake pedal when the engine is off but as soon as I turn the engine on the pedal feels like I havent even bled the brakes yet.Master cylinder is new.Could this just be air in the system still?

 

[threlfall92]

you might have blown a brake line. when the engine is off it might feel like theres pressure but there isnt. you can tell if its a brake line if the pedal goes right to the floor easily then grabs really good.

 

[junior]

If you have ABS, you may have air in the antilock brake module. There are a series of valves in there that adjust pressure to each wheel and it is almost impossible to bleed this unit at home. You may need to have a shop connect a scan tool that will move the valves to remove the air.
You can try to bleed the lines at the master cylinder. Have someone pump up the brakes while you crack loose the brake lines one at a time where they connect to the master cylinder. Air will always find the highest point and the master cylinder is it.

 

[mike d]

did you bench bleed the master cylinder before you put it on? you have air in all your lines