Replaced my front right caliper today. My 4Runner is a 2007 that I've owned since 2009.
My first replacement was in April, 2014 with lifetime warranty calipers from O'Reilly Auto Parts. Both of the original calipers had both inner pistons totally seized. They were bad enough that I had to beat the pins out with a hammer and punch to get them off.
One of those 2 calipers installed in 2014 is still working fine. But the right front failed for the second time in October 2017 (3.5 years, ~35-40k miles later). It just failed on me for the 3rd time (11 months, 7k miles later). The second and third failures were both one of the inner pistons.
This is just ridiculous. I used to live in the northeast - which probably helps to explain the first failure of both calipers. But I'm now in the pacific northwest and there really isn't a salt issue here. I just don't get how Toyota can make such garbage calipers. These things have to be the worst design possible.
Best advice I can give anyone reading this is to buy calipers locally and with a lifetime warranty. I can change these things blindfolded now.
By the way, I regularly remove and re-lube the pins in these things in an attempt to keep the pads moving properly. This makes almost no difference, from what I can tell. If they are rusted solid, the pistons will freeze for sure. But it turns out that even if the pins are coated in caliper grease, the pistons will freeze...