Intermittent soft brakes

[mamu]

I have a 2006 gmc 3500 dually. Ive been battling brakes on this thing for the 3 years I've owned it. It got new rotors and pads in the rear right before I bought it. When I bought it the line from the abs valve to the rearend had come apart and was leaking. So I built a line for it and put it all back together. New calipers on the front. Another new set of pads. New lml hydroboost. The brakes are soft soft soft. Some days I can brake real hard like emergency nearly hit the ditch thinking I'm about to kill everyone around and they start working good. Full pedal. Just touch the pedal and feel them working. Then as time goes on they get soft again. I've put the kryptonite stainless lines on the front. Have the rear ones just haven't got to it yet. Was thinking maybe rear calipers next? Truck has 315k on it. Not sure about the power steering pump. But steering is great on it and doesnt get tough when I'm braking and turning.

Any thoughts on where to go next?

Thanks

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[Awenta]

Did you bleed the abs pump when it drained out? Did the master cylinder ever drain out?


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[mamu]

I have bled everything and even ran the abs automatic purge with my scan tool. Dont think the master cylinder ever went dry. Just low.

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[DAVe3283]

If the brakes are soft after bleeding properly, and there is no loss of fluid, it's almost certainly the master cylinder. The one on my truck went out sometime around 300k miles. A new one is $75, but I was cheap/dumb and got one from a junkyard for like $15. Still working, so maybe not too dumb of an idea? But for only $75, I'd recommend new.

 

[gmduramax]

My truck does the same thing. New master, booster, fluid everything bled. My pedal will be decent but if I hit them hard and drive around it will be rock solid for a while and will go back to being softer.

 

[bmc1025]

Check all of the caliper pins to make sure they are free and lived my rear were stuck causing the same issues. Now I'm having problems again but I am going to upgrade the booster and MC due to some leaks and I'm at 275k