Aecer

1990 4Runner 3VZ 285+K miles with brake issues - long throw on brake pedal, too spongy .

A year ago, front caliper pistons froze up. Replaced caliper. Could not bleed the brakes using the normal person inside pumping pedal, person outside open/close bleeder. Inspected brake master cylinder - corrosion inside. Replace MC with rebuilt unit. Still could not bleed properly.

Careful inspection of replacement MC showed that the sliding cylinders inside the MC never were pushed far enough towards the rear by the MC spring to uncover/open the hole in the MC that allows fluid from the reservoir into the brake system. During attempted bleeding, the MC spring would push the cylinder to the rear, producing suction on the brake system, but would never open to let any new fluid into the system. Could not gravity bleed for this reason either.

Returned the MC - they agreed it was defective and replaced. A replacement was provided, and it seemed better, particularly when fluid was in it. First question here - does anyone have a 4Runner master cylinder they could look at to confirm that the spring does push the cylinder inside to the rear sufficiently far to uncover and open the hole that leads from the reservoir into the cylinder?

Ultimately, this 2nd MC was installed, I built a pressure bleeder and bled that way. IIRC, I applied slight pressure at a wheel, which may have moved the MC cylinder to open up the port to the reservoir, then applied pressure from tank down to bleed. I've used it for a year since this (not continuously), but have never been happy with the throw (pedal moves too far towards floor). It has slowly seemed to get worse. There is no apparent leakage in the MC (continuous pressure doesn't seem to decay to floor), but very hard pressure will now reach the floor.

A few days ago I decided to adjust the pedal (inside cab under dash) to give a bit more pedal throw. It seemed to help a bit. I drove it for a few days. Then the wife drove it and came back with the pedal at the very top of the throw - brakes were now hard as a rock and slightly dragging. This was a huge change. Slightly touching the brake gave near-instant hard braking. I should mention that this happened simultaneously with a broken steering damper that caused severe shaking of the vehicle (nut had fallen off damper bolt). Because the brake was now hard as a rock, I concluded that the previous problem could not be air in the system or a bad bleed. Second question: do you agree with this? Third question: I suspect that the shaking caused the cylinder inside the MC to move farther back and allow brake fluid into the system. Does that seem likely, or something else?

I test drove it in the condition described above, and braking was great, but at the end of the drive, the brakes seemed to be slightly dragging and slightly hot. I readjusted the brake pedal to original and it is now back to the spongy-bottom of travel condition.

Last question - Would you replace the MC? Buy a 1" upgrade MC (current stock MC is 7/8") Anything else?

If you made it this far, thank you.