This all on the front. But otherwise, that's correct. Vise grip on both front hoses just before they go into caliper results in a hard, not sinking pedal. Take the vise grips off, pedal slowly sinks until its 3/4 of the way down and is very spongy at that point.
One observation I made was that it seemed like the calipers were "returning" too far. When someone pumps the pedal with the truck running, I can see the piston suck back in more than I am used to seeing. I can take the calipers off just by hand which also seems unusual to me.
I was wondering if maybe there is a certain point in the master's travel that it would not hold pressure, but would still hold pressure at the beginning of its travel. But a bad master would not hold fluid under any circumstance I would think.
I will give the c clamp all the way down trick a shot. Good thinking.
