'72 2wd Blazer brake issue

[jda1a]

My '72 2wd Blazer brake warning light came on last night. Checked master cylinder, rear resevoir empty. Checked under truck no leaks at wheels. Bad master cylinder, or something else? Truck has dual master cylinder power brakes with front discs. Thanks for any help.

[umwtnt]

When was the last time you checked the fluid level? The rear reservoir is normally for the front disc brakes. As the pads wear out the fluid level goes down to compensate for the wear. My first though is to pull the front wheels and check the pads and look for leaks on the calipers. Good Luck.

[PHAT TONY]

X2 on the pads; that's a big caliper to fill.

[Longhorn Man]

Even if the pads are worn to the steel plate, that would not drain the master cylinder.
Inspect the system for a leak. Look on the back side of the rear wheels, if the ntires are soaked with some kinda oily lookin fluid, that's the brake fluid.
if no leaks are found anywhere, then the master cylinder has a leaking seal, and it is going into the booster. Time for a new master cylinder. (very cheap at any parts house) If you have the money, it'd be good idea to do the booster too. The brake fluid will eat the rubber and pop the diaphram in there. It may not be next week, maybe not even next year... but it WILL happen.

[jda1a]

I think you may be right about the master cylinder and booster. I checked all four tires / wheels , found no leakage. Pulled the front wheels, still have 3/8" of pad left and no signs of a leak, Filled the rear resevoir with fresh fluid then pumped the brake pedal several times and the light went out. Will drive and re-check. Thanks to all .