Brake Booster or Master Cylinder ISSUE?

[JosGon]

I have a 86 K1500 just rebuilt the rear drums and used calipers to measure distance. Bleed all the brakes and still have alot of mush it stops but now well. Fast forward to today brakes went out completely after a few pumps of the brake they tightened up a bit. Could this be a master cylinder or booster issue? I hope so because I just ordered a new one of both. I'm trying to make this bad boy safe for my son to drive he will be getting his license soon. Thanks for any info in advance.

 

[Blue Ox]

Bad booster will make it feel like you're trying to crush a garbage can with your foot. If the pedal sinks easily it's a hydraulic problem.

I'd start by bleeding more. If you just did work and now have a problem I'd be looking at the work first.

What about the slack adjustment?

 

[RaisedK5]

If the rears are adjusted out properly and are not the problem, then I would bet on the master. Sometimes the bore in the master can have crap in it that doesn't affect anything ........untill you bleed the brakes. Then you move the piston past where is usually lives, get into the crap which messes up the seals and you get to put in a new master.

 

[Grit dog]

What does “used” calipers mean? Are you certain the shoes are adjusted out far enough? That could give more pedal travel.
Did they work before and you bled because you replaced the wheel cylinders?
What about the front?
Honestly sounds like still air in system. That’s the symptom when you let it sit and the pedal goes to the floor again, but pumps up eventually.

Or what K5 said above.

Nice truck! Your kid must be pumped!
Our youngest turns 16 in 2 months and his truck is still in 100 pieces…lol.