Have a '97 Limited that sat for about 10 years from the previous owner. It has about 215K miles on it now. The previous owner was a local parts salesman and the 4Runner was his daily commuter around the region, so almost all the miles are highway miles, if that matters.
Anyway...I replaced the front rotors and calipers when I first got it due to some warping on the rotors and decided to put the Tundra calipers and rotors on the front, but only replaced the pads on the rear. There were no indications of leaks anywhere on the braking system.
My wife complained yesterday that the brake pedal felt weird and wanted me to drive it.
The brake is usually right at the top, but sometimes would go further in its travel before the brakes grabbed. This morning while I was driving had the same issue, but as I pressed the pedal further a rear wheel locked up and it dragged a few feet.
Are the brakes a true dual-circuit? I think that maybe the front circuit bleeds past on the master cylinder when the pedal is pressed slowly. It only did it to me once and to my wife yesterday the one time, but it is scary when it happens. The reservoir shows no sign that fluid has leaked and if you allow the pedal to travel back up, the brakes are right at the top again.
They have never felt "squishy" after replacing, so I feel like they were bled properly when I replaced the calipers.

Are you sure?



The master cylinder has an extra chamber that works with the rear drums to improve pedal feel. If you don't follow the instructions above, there will be feel/travel issues with the pedal. Also, if you let the fluid drain out when you did the calipers, you can introduce bubbles to the ABS module - which isn't easy to bleed (gravel road trick, here you come).

So, you have a few things to check.

- Master cylinder "bench bleed" using the FSM procedure.
- Normal bleed according to the above order
- Activate ABS on a gravel road or similar
- Re-bleed with above procedure, repeat as necessary.

You may also want to make sure the rears are adjusted correctly and that you ALWAYS use the parking brake - that is what adjusts the rear drums.

-Charlie

After removing the old calipers, I placed caps on the brake lines to prevent excess fluid from leaking out. I feel like that air would not have made it to the ABS module or master cylinder. I also made sure the reservoir never went low while bleeding.
After installing the new calipers and pads, I followed the brake bleeding procedure to remove the air from the new calipers. After replacing the calipers and bleeding down brakes were at the top of the pedal, as they still are.
It's only occasionally that the pedal travels further than expected before the brakes "grab"...and only yesterday started traveling so far that the front didn't grab and the rear were the only ones working.
I don't think my wife uses the parking brake at all as it's a very flat area where we live (no hills). They were adjusted properly after initially replacing the pads and calipers.