No rear brakes - troubleshooting tips

[CombatYoga]

I run RBF600, and recently changed my fluid out after a nasty mud race.
The sand and dirt completely ate up a set of brake pads, and the fluid
when I changed it out was very dark colored.

After changing the brake fluid out, I went for a ride with a friend in some pretty technical terrain. The rear brake was not behaving - I had to really stand on it to get the rear wheel to lock up. I chalked it up to air in the line and rode the rest of the day.

Tonight I used a syringe to force brake fluid in from the bleed nipple up to the master cylinder. This has been my go-to method for getting air out of a clutch or front brakes, but admittedly the master cylinder on those is much higher than on the rear brake system.
After doing this and putting things back together, I no longer have any rear brake action at all. I've pumped on the rear pedal a good bit and I'm not getting anything. The pedal feels like it's dead - like there is no back pressure.

It's late so I'm giving up for the night, but my first thought is to see if any brake fluid pumps out of the bleed nipple when I work the rear pedal, i.e. do the normal bleed procedure. If no fluid comes out, then I think that tells me the master cylinder is malfunctioning - maybe a seal?

I have a race this weekend and family coming to town, so time is very limited.

Any troubleshooting ideas?

Thanks!

 

[Boom Boom]

Try pulling the assembly from the disk, work the puck out to max travel or use some air and pop it free. Clean everything very good and then reinstalled the puck but do not push in any further than needed. Remove the bleeder nipple and inject oil into the area behind the puck making sure to be 100% full and then work the puck in and push out some oil to confirm no air. Reinstall bleeder nipple and do a quick bleed.
I went nuts bleeding a system and had no peddle only to realize I had air in the chamber of the puck and doing a standard bleed either pressure or vac will not fix the problem if a puck has air in it.