this is the part that I don't get: don't I have to get the caliper off to replace it?

If I don't disconnect the brake lines to replace the caliper, I don't have to bleed my system (yet)?

 

See where the two guide pins and boots are? The caliper is called a sliding caliper whereby one side moves independent of the hydraulic piston. As the pads wear, the piston extends from its bore AND the sliding caliper moves towards the piston.
The caliper should slide side to side on those pins using light hand pressure. It should also pull apart for cleaning and greasing too pins. If it won't come apart at the guide pins, it needs replaced.

Here the pins are cleaned and ready for grease and here is a simple tool to compress the piston without binding it or marking the caliper. Insert it into used pads. Last pic is completion.

 

So if I break it apart and clean the caliper and lube it and it still won't slide easily, then I need to replace?

Just trying to do the least amount of work here. I already have the right rear caliper but I don't want to bleed my brakes or buy new rotors at the moment.

I see that the rear pins are a common failure point on IS250. Now that the cars are older and older, makes sense. Plus heat from braking

BTW--do AWD and RWD use same rear brake calipers?

 

250 300 350 all use the same design and one is no better than the other. It's the pad slap that gets this design in trouble. An extra 20min per side to grease the pins and maybe replace the rubber boots to keep the water out and they are fine.

But ya, if it won't slide by hand, it's not usable. Bleeding the rears takes less than 10min with a helper. It's really not that hard. When filling empty calipers, keep filling the MC as you don't want it to run dry or you pump air in and have to start over.

Communication with helper is all you need.
*You say down. They say down when pedal has pressure/is down.
*You open bleeder, seal it when flow stops.
*You say up, they let up.
​​Repeat loop until no air comes out. Keep filling MC. Then do the other side. RR first. Then LR.

That's it. Basic.... use only new fluid from a new container.

 

I did the same thing after replacing the pads and rotors on my mom's camry. My brother and I went to the extreme with new fluid and bled each caliper until the fluid changed from dark to light topping off the reservoir as needed. (LHD) RR, LR, RF, LF - I understand if its RHD it changes to LR, RR, LF, RF but I may be mistaken

Now a 20 year old camry brakes like new and I know there is no water in the fluid.

 

I ended up replacing both rear calipers (incl brackets), rotors (sucked since old ones had EXACT same mm/thickness as new ones) and pads.

About $175 in parts and $160 for a mobile mechanic to do the labor.

Didn't want to waste time rebuilding calipers nor using heat/hammer to get the seized bolts out.

For any IS250/IS350 owners out there, PLEASE GREASE YOUR CALIPER PINS annually on one of your tire changeovers etc. Mechanics can do it for $20-$60 and will save you lots of issues later. You need tires off to see inner brake pad wear (something I knew but ignored till was too late).

My only consolation was getting parts cheap but $10 caliper grease if you DIY.

Thanks for all the responses in this and other threads btw. You guys are all lifesavers!