The brakes on my truck had become very soft and airy sounding, so my friend offered to fix them. He loosened the bleeders to gravity bleed the truck and left them open too long and all the brake fluid drained out. I filled up the reservoir and bled them properly, but there was no pressure buildup whatsoever. I suspected the master cylinder may have gone bad so I replaced it with a new one and bench bled it before installing. After installing and bleeding there is a tiny bit of pressure, but it’s almost nonexistent. I’m completely at a loss. Any advice would be appreciated.
What year?
Some ABS systems require a scan tool to properly bleed the brakes when the brake system is opened.
Tester
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- I’ve heard conflicting statements about that. Most people have told me that it shouldn’t need a scan tool, but there are a few who said I did.
Those are the people to listen to.
Tester
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Abs likely. Since mc is new there is either air in system or movement in brake part somewhere. What happened just prior to this? Any work done? How did you bleed system this last time after mc?
Tester
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There was no work done prior. After installing the new MC, I gravity bled it first, then went to each bleeder and had a buddy pump for me.
So the fluid draining completely out let a bunch of air into the modulator? 
Ya think?
Tester
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Did the brake warning light came on?If yes,I would check all brake lines since rust could be a problem on a 17 year old truck. After bleeding it,ask someone to press the brake pedal while you look under the vehicule for a leak. Since your vehicule is equipped with drum brakes in the rear, it is difficult to find a leak in this area unless you take out the drum housing.The most common place for a leak is at the wheel cylinder.
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