First have to say glad to be back here. It's been a long long while.

Now the issue. My 2000 Ram 5.9 4x4 reg bed/cab sat for about 3 weeks no driving. Before it sat the brakes were fine. Then I decided to dust it off and let her out. Took a drive to the store, no issues. On the way home I noticed the brakes got real soft and didn't stop like normal. I get home and check the brake fluid, almost empty. Top it off and comeback later. Brakes are back to normal. Fast forward a week. Start it up press the brake to shift, pedal goes to the floor. Hear a clicking from the master cylinder. The ABS and brake lights both come on.

I'm stumped. Checks for spots for fluid none. Check each caliper, none. Checked the lines, okay. Where could it have gone?

 

if you lose brake fluid from the master cylinder after recently refilling it, then you have a leak, so don't drive it until fixed. a common leak point is the brake line running above the gas tank (from salt/rust). other potential places to find leaking fluid are at the front calipers or rear wheel cylinders.

if you are not losing brake fluid, but the pedal goes to the floor - then its likely a bad internal seal in master cylinder, caliper, or wheel cylinder.

one last option of lost fluid is the vacuum booster. i don't know if its possible for fluid to get in there or not. i doubt it, but the booster does have vacuum on it.

 

I'd bet on the leaky rear metal brake line and/or rear wheel cylinder(s), especially if none of that has been repaired before.

 

For me it was the seal on my master cylinder, and leaked into the booster.
Had to take the booster off to drain all the oil and mop it out as best I could... hopefully I got it early enough and the diaphragm wont deteriorate due to the oil exposure.