Where'd my brake fluid go?!?!?

99 Z71 5.3 4x4 Ext cab 301k

Felt an odd pulsating and slowwwww loss of pedal pressure over a 24 hr period. Before I could get into it, the pedal goes to the floor. Brake idiot light is on and fluid res is down significantly but I can't find where it's going. No codes being thrown. No puddles or rivers anywhere that I can see. Just took a pint of fluid and still not up to the max mark. Based off of the VIN, dealer says that I've got vacuum boost. Have a single rubber hose running from the booster to the back of the engine behind intake cover. Brake master has never been replaced. With key off pedal DOES create some pressure after 2nd pump but disappears as soon as engine is turned on. ANY thoughts beyond "Dude, you're just screwed." ?

if fluid is still going down you either have a leaking wheel cyl/caliper or your master cyl is leaking in the rear and getting sucked into brake booster or one of your rubber hoses is leaking. the brake light could be on because fluid level is low or the system is detecting a unequal pressure from front to back. if fluid is not still going down that would indicate your master cyl is bad. if it is still going down will need to pull the wheels and looks for signs of leaking or have it towed to a shop for them to check it for you

THE LOSS OF BRAKE FLUID WOULD BE MOST TIMES A LEAK ! Now hard brake pedal is a bad leaking booster. leaking meaning it cannot hold a vacuum so you have NO power brakes. just to be sure on brake fluid being sucked into the intake manifold simply remove that hose to the brake booster see if its with brake fluid. if not then you have a leak.. that is on your vehicle MY guess the rear brake line that runs next to the fueltank and frame rail.. many with this vehicle made back then have had both brake and fuel line failures due to cheap metal lines used my GM. I too had this mysterious leak... very difficult to see .. running a new line is also a PITA..

OK... Found it. Left rear section between the lead in to the caliper and the T fitting feeding both sides. All of about a 14" section of line. Small enough leak and close enough to the rim that the rim was catching it and just spinning it off. Hence no puddles or rivers. Now I just have to figure out the connector size, preferably w/o removing the line section so I can GET to a shop to get the replacement part.

OK... Found it. Left rear section between the lead in to the caliper and the T fitting feeding both sides. All of about a 14" section of line. Small enough leak and close enough to the rim that the rim was catching it and just spinning it off. Hence no puddles or rivers. Now I just have to figure out the connector size, preferably w/o removing the line section so I can GET to a shop to get the replacement part.

so it is the metal line on the driver side near the caliper hose ? what you can do if the metal line is bad is re-use the fittings... buy new metal brake line. put fittings old on then flare the ends... GM uses 30 deg flares not 45 deg flare. you will see that when you remove if its still oem..

Yeppers. Small section with the spiral reinforcement wrap. Cut the line at the leak, crimped it and pulled a fitting to match it up. Fixed for all of $10 between the new line and a quart of brake fluid. 20" piece of straight, pre-flared/fitted line did the trick. Wasn't able to crack the bleeder, though, despite the PB Blast bath and didn't want to risk shearing/rounding it today as we're supposed to dip into the 20's over the weekend. Bled the new section of line, at least, so there's only about 6" left and things are at least functional.

Only thing that was different was the old, assumedly OEM connector, was 9/16 and the new aftermarket is 3/8. Bleeder screw is 10mm just for the record.

Yeppers. Small section with the spiral reinforcement wrap. Cut the line at the leak, crimped it and pulled a fitting to match it up. Fixed for all of $10 between the new line and a quart of brake fluid. 20" piece of straight, pre-flared/fitted line did the trick. Wasn't able to crack the bleeder, though, despite the PB Blast bath and didn't want to risk shearing/rounding it today as we're supposed to dip into the 20's over the weekend. Bled the new section of line, at least, so there's only about 6" left and things are at least functional.

Only thing that was different was the old, assumedly OEM connector, was 9/16 and the new aftermarket is 3/8. Bleeder screw is 10mm just for the record.

the fitting is different that is why I said to use the old ones [if good]... on the bleed of the caliper using a box wrench 6 point try to loosen... if its too tight then loosen the hose connection to the caliper have someone work the brake pedal as you loosen and tighten the hose connection .. not perfect but it does work .. also pushing the cal piston fully back as you open this hose connection will drive out all the air bad brake fluid...using C clamp.. since this line has these issue you best check the line next to the fuel tank/frame rail area .. many have had this fail more than once.. they do have at some stores better lines that can handle our salted roads.. not the kind of job to do this winter.