99 rear disc brakes not working

Recently noticed pedal travelling to the floor while driving. Turned off and sitting for a while pedal goes once to the floor. On the second push the pedal is firm and where it should be. When driving the pedal gets a little better if I pump brakes while applying. Noticed when I tried stopping on snow that only fronts were applying as the rear wheels kept turning. Rear has disc brakes.

Possibly air in line, or master cyl on it's way out? To cold today to try bleeding. Note, also posted on Blazer Forum for help.

If the dash "BRAKE" light comes on, possibly seals leaking in the master cylinder, or there is air in the hydraulic system. Your vehicle has ABS, if an air bubble gets into the ABS unit, (master cylinder ran dry etc), you must use an ABS capable scan tool to bleed the system. Using the scan tool is the ONLY way to open the solenoids in the ABS unit to bleed air out of it.

The Brake light is not on. Was thinking I would try bleeding them on the weekend to see if it makes a difference.

Thanks

If the brake light is not on, and hasn't come on in the past, and the system has not run low on fluid, you can bleed it the same as a non ABS system, just don't let the master cylinder run dry. If the pedal still sinks to the floor, you most likely need a master cylinder.

If the brake light is not on, and hasn't come on in the past, and the system has not run low on fluid, you can bleed it the same as a non ABS system, just don't let the master cylinder run dry. If the pedal still sinks to the floor, you most likely need a master cylinder.

Thinking I have a bad master cyl.. Bled starting at RR and went through about 6oz of fluid before air bubbles stopped. Then notice fluid stopped as well even though son could push pedal to the floor. Went to bleed LR and broke off the bleeder so tried at the line. Fluid would not spurt out when son pushed the pedal. Only dripped out, and very little at that. When fired up the truck the Red brake light is now on. When brakes applied the rear brakes don't even move. Tried road test and problem is worse as pedal now goes to the floor before the front brakes apply.

See post #2. Keep in mind a bad proportioning valve could cause this also.

See post #2. Keep in mind a bad proportioning valve could cause this also.
Went back to have another look and there is a proportioning valve attached to the abs unit. Checked and master cyl is pushing fluid into valve. Checked for output and no fluid coming out of the unit assembly to the rear brake line.

How do I reset the valve or do I need to go to a shop that has the scan tool?
Or can I purchase a brake scan tool and be able to do it myself?

Also - not sure what you meant by see second post. Is it the second post within all this dialogue?

Update - Removed end cap from prop valve and it reset after I removed and replaced the plunger. Then bled the air from the master to the outlet side of the abs unit going to the rear brakes. Now have front and back brakes but way too much pedal travel.

Guess next step is to buy more fluid and rebleed entire system, both front and back. Should I be using Dot 3 or Dot 4? Does it make any difference?