I wanted to document this as I had a helluva time finding threads on all sorts of toyota forums that had the exact problem.
My car is the newer vvti LS400 but from Japan where it likely sat a while.
The brake pedal went to the floor but then stopped the car, it was pretty terrifying initially, stopping power was also pretty low.
I tried replacing the brake booster, which turned into 6hrs of cutting it out of the firewall, did nothing.
brake bleeds of all sorts, adjustment of pedal, notta. I bled the master cylinder with the fingers-over-ports method after it was on the car (bench bleed would be easier)
What the problem was, was pistons sticking in the front calipers. I read that if you clamp off one of the brake lines with vice grips and the pedal goes firm, you've got a caliper problem.
Pedal got harder with LF clamped but really firm with Both clamped.
Took off calipers, boots and seals looked brand new but the most of the pistons barley moved in their bore. After using compressed air and c-clamps (to hold in the pistons that were free in order to keep the pressure up to blow out the stuck ones) I removed each piston seal, greased it with marine grease and then lightly greased the bores (as per LexLS.com caliper rebuild) Pistons were super clean already but I sanded them lightly with 1200grit, now they move very easily in the bores.
Then a normal bleed on the calipers with a person pumping inside until air bubbles were out.
However, the brake pedal up on jack stands did not get firm, UNTIL I took the car out for a drive, as soon as I moved the wheels the brake pedal went firm. I was going out to try and his the ABS on a gravel road to clear air in there, but this proved not necessary. I've been at it for months trying bleed after bleed, this and that, and there you are. I read on 4Runner forums that people had the same problem with their 4-piston calipers, apparently the Seal rebuild kit is avaliable through Lexus or toyota, way cheaper then getting a rebuilt caliper.
I was ripping my hair out on this one, hopefully this post helps some people who search it in the future.
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Thanks for the write-up and glad you found the cause. Have had similar experience on other cars where a caliper does not have smooth travel and is stuck. Same routine, bleed, check booster, master cylinder, caliper and the light bulb goes on.
any car is susceptible if the fluids are not changed within a reasonable time frame..humidity is more of a factor than heat itself. brake fluid is hygroscopic..dot 5.1 is more so than dot 4..and 4 is more so than dot 3. never use dot 5 silicone based fluid.
It was a new experience with me on 4-piston calipers, I had them as well on my 3000GT but never had this problem.
I've had the car since Dec of last year but have been fighting with this problem since April so I can get the car safetied and legal on the road, finally finally got it!