Low spongy pedal

2010 Tacoma.Low brake pedal.New pads on front.Rotors and calipers are fine.Rear brakes are like new.Wheel seals fine.Rear brakes are adjusted up tight.I can stop but it's got a really low pedal and not hard.Sitting at a light I can push hard and pedal will slowly sink to the floor.We bled brakes three times.No hard pedal.Thinking it's the master cylinder.Want to take the lines off of it and bleed it then the whole truck brakes again before I get a new master cylinder.Im just not sure if anti lock brakes should go all the way to the floor when pushed like these do.Whats your thoughts.

 

Have you tried Stainless Steel brake lines? Maybe one of your rubber lines has a bulge and is flexing.

 

If you give short rapid pumps does it rise/firm for a moment? If so, its the master. The falling pedal is an internal 'leak' or bypass where it cant hold the pressre. In fact, it is very difficult to bleed a system with a failing master in the traditional 2 person method.

Its not a wheel line. A a line so soft that the pedal would drop would fail, and the opposite wheel on that axle would grab and snatch you sideways

 

Yes that's what it does.I can pump quick after the first hit on the brake pedal and it will come up and feel a little better.I have had this problem before on an older truck but I wasn't sure if the antilock brakes would play a part in this going to the floor.Dont know enough about this but thought it was a failing master cylinder. Another question about bleeding.I read about the proportioning valve needing bled.Is this true and where is it? Thanks