It is possible the master cylinder is shot.
If it were good before, with a full pedal, Then suddenly it was going to the floor, the piston traveled into the "not used" portion of the cylinder (maybe it's not smooth inside anymore/ buildup/ basically 'barnacles' eating on the rubber piston) (OR NOT!)
If you let it run out of juice in the master cylinder, it still may have air trapped in it. As I showed in the previous answer, 2 people can "Bench Bleed" on the vehicle. I really like this method! The tubes and adapters needed are cheap at an auto parts store. No need in pumping fast. I sorta pinch the tubes with my fingers sorta like a 'check valve' (fluid/ air out into the reservoir-won't back up). This makes it go faster! (the air can only come out, not move back and forth in the tube) This back pressure usually resulted in blowing the tubes off of the adapters. EZ fix! Wrap 2-3 wraps of string tight over the tubing, over the adapter.
There may be better videos on you tube, I liked this one (in previous answer)
Another possibility for a low pedal/ a pedal needing 1/ 1-1/2/ 2 pumps to get it high and tight each time it's used, may be the brake shoes need adjusting closer to the drum. Sometimes the self adjusters get dirty/ rusty and quit adjusting. You may have to replace/ remove and clean/ or use your brake spoon to get 'em closer to the drum surface.
What happens is the 1st pump may take you closer to the floor. This gets the shoes to the drum (pushing a lot of brake fluid to travel that far) The quick pump, which gives you the higher pedal (2nd pump) now only used a little fluid, as you "grabbed it" before the brake shoe springs could have time to shove the brake fluid back into the master cylinder from the 1st pump. Adjusted out properly only uses a little fluid to move the shoes just a little bit, resulting in a higher pedal all of the time. Calipers on the front end do not have this kinda problem.
Did I make this understandable?
Thursday, October 20th, 2016 AT 7:46 PM