Early this summer I rebuilt the front calipers on my 4Runner, and I noticed that despite the fact that I'd flushed a large amount of fluid through the system, the calipers still had some seriously nasty fluid inside them, probably no coincidence that 5 of the 8 pistons between them were seized. I'm currently in the process of doing the same on a pair of Subaru front 4 pots, which have virtually an identical design, and, IMO, the same flaw. These calipers have 2 opposing halves, with 2 pistons in each half, and a fluid passage between the halves at the top and bottom, so there's a relatively large volume of fluid in them. Yet, the fluid inlet and bleeder are very close to each other, meaning when you flush the fluid, it only actually moved past one of the 4 pistons. Here's just the inner half with the bleeder removed. 2017-12-12_03-37-15 by Numbchux, on Flickr Of the 4 calipers I rebuilt this year, every one of them had the inner, lower piston badly seized worse than the other three, and required several days of electrolytic rust removal to release. What does this all mean? Well first, if you have these on your car, I'd recommend removing, compressing the pistons, and thoroughly draining the entire caliper, maybe whenever doing new pads (which is counterintuitive since these calipers don't need to be removed to replace the pads). I'm considering modifying the calipers on my 4Runner to help remedy this. These calipers all use the same casting regardless of side, it's only in final machining that they become different for left or right side. This means there's a spot cast in the housing for an inlet in front of both inner pistons. It would be possible to drill and tap the lower one, and plug the upper one so fresh fluid is introduced right into the vulnerable cylinder. The tapping would have to be perfectly square so the banjo fitting would seal, but I think it's doable. Ideally machine that face perfectly flat on a mill...
I'm guessing there is only one bleeder on those? And I'm also guessing that is why Brembo's (and a lot of other floating style calipers) have two bleeders?
I haven't had one of those apart, but with these Subaru ones and the Toyota ones I have, the passage between the 2 halves is routed such that the bleeder goes into that channel right at it's highest point, and therefore can bleed air out of both sides simultaneously. I suspect that the 2 halves aren't connected at all on the top, many Brembos use an external hard line to connect the 2 halves on the bottom. The Subaru brembos still have the fluid inlet on the upper half of the caliper: But if my guess is correct, than using the outer bleeder would flush fluid through the whole caliper. Whether that was actually how it was designed, or a side effect of some other design feature, I'll probably never know. There has to be a reason for it, though, here's a Z32 300ZX Caliper, exact same basic design....
