if i took off the front calipers, and left the rear calipers untouched, do i need to bleed all 4 brakes, or just the front 2? tia...

 

When you crack open a bottle of brake fluid, it has almost a ZERO shelf life. So use up the whole bottle when bleeding.

To answer your question directly, I believe you will be OK just bleeding down the fronts.

Get a turkey baster from your local Wal-Mart for $.99 and remove as much fluid from the master cylinder as possible. Then re-fill the cylinder with fresh fluid and bleed till the fluid is clear.