Interminently Soft Brake Pedal

I'm in need of some insight! I've exhausted the extent of my knowledge and the limits of the search function here on the forum.
Here's my situation:
About three weeks ago, I noticed that my automatic '08 TRD off road 4.0L was having an occasionally soft brake pedal. I still had plenty of braking power, but the pedal sank twice as far to produce a stop. About a week after I first noticed the softness, the pedal began (randomly) sinking nearly to the floor without producing a complete stop. Three to four quick, short pumps firmed the pedal and produced a normal feeling stop. I inspected all brake lines for cracks or rust and they all looked great. I checked the booster for vacuum and any fluid that may have seeped from the master cylinder and it was fine.

I replaced the master cylinder with an A1 Cardone reman. from O'Reillys. The part was bad out of the box and wouldn't hold much pressure and produced an even softer pedal than the old MC. While I had the wheels off to easier bleed the front lines, I went ahead and replaced the brake pads. I ordered a new Centric master cylinder from autopartswharehouse.com and installed it. On both of the master cylinders I installed, I bench bled them (effectively as far as I know. The Centric master is giving me the same problems. Probably a third of my brake pumps feel soft and a tenth nearly hit the floor. Thinking that some air may have gotten into the ABS unit, I found a dirt road and hammered the brakes initiating the ABS in forward and reverse about six or seven times each. Then I re-bled the system, passenger-rear, driver-rear, passenger-front, driver-front to no avail. I still have a soft pedal! Also, I have bled the system with the key in the on position and without it. I have used the old fashioned method of bleeding, and ive use a vacumm bleeder. Nothing seems to be helping. What have I done wrong or missed?

 

you might try checking and adjusting your rear brake shoes. adjusters do freeze up sometimes. too much shoe clearance equals long pedal travel

 

I just replaced a wheel bearing that was starting to go bad and my brake pedal is much better. Prob not your problem but always good to check.

 

Hey Babur, I've had the exact same problem for about 6 months and almost blew through a stoplight getting off the freeway today. Finally figured it out. It's related to the load sensing valve overloading the rear cylinders and causing a tiny leak that allows air in and out of the system, but is too small to loose a noticeable amount of brake fluid. Most likely a result of the very common rear leaf sag that occurs in most tacomas. I won't go into too much detail because I just posted a detailed explanation in another thread. Check out "recommendations for mods to tow with a gen 1 taco" hope it answers some questions

 

^^ Second gens don't have a load sensing proportioning valve.

And yeah, check your shoe adjustments and front wheel bearings for play.

 

There may be a specific procedure to bleeding just the abs unit. I'm not sure about the tacoma. On some vehicles when air gets in the abs unit, simple bleeding doesn't work.