Q: Cannot flush the brake system

I have a 1985 Ford Mustang, 2.3l with only 29,000 miles. I am trying to flush the brake lines.

I have a small vacuum pump as can be found in any auto store. With great effort I was able to flush the right rear (drum) brake. However, although all the other bleeders are free, I cannot get any brake fluid out of the other three wheels (front disc) although the pump shows about 15" of vacuum. The pedal is firm, with no signs of air in the system, nor does the car pull to either side.

I removed the bleeder on the left rear wheel and it is not clogged.

For what it's worth, I drove the car after filling the master cylinder with new fluid and flushing the right rear. Now the front brakes (large) reservoir shows some old fluid, but the small rear brake reservoir still appears clear to me. I don't know if that means something is amiss with the rear brakes, but in any event that wouldn't explain why I can't flush the front brakes.

I'd appreciate any advice you can offer.

My car has 29000 miles.
My car has an automatic transmission.