Brake Bleed; Does it clean out the caliper pistons?

I'm curious about the brake hydraulic system, direction of flow, and where does it flow? I want to do a full brake line flush as the res is kinda dark and I was hoping I could save myself a future caliper replacement or rebuild. Does the caliper piston get flushed with new fluid when you do a normal bleed? I imagine the whole system recirculates which is why it gets dirty over time. It would make sense then that the bleeder valve is lower than the pistons, is that correct. And how do you bleed the return so you don't dirty your new fluid in the res?

In general I hear and see people bleeding the brake line after a brake job. I'm not sure but I guess this has something to do with charging the master cylinder such that your brake pedal applies the correct pressure while being in fully maximized starting point.

These sound like noob questions because I have only recently begun to do all my own work on my Taco but the work is not that difficult if you find the right YouTube Taco channel and follow steps. e.g.Timmy the Toolman, Toyota maintenance, Faye Hadley etc. I have a logical mind just lacking the mechanical experience on brake bleeding at the moment. I am waiting on new pads and rotors for the front.

I got plenty of tools. I have successfully done all my fluids, oil, transmission, rear and front diff, transfer case, grease the u joints, new spark plugs, replaced all interior and exterior lights with LED bulbs and upgraded with TEQ customs LED headlights. I am new to it because I have time to do it now and enjoy working on my Taco myself.

Thank for your effort and time for help.

 

Bleeding the brake lines removes air bubbles, which can cause brakes to fail (not stop, freeze up, seize a wheel ... other unpleasant things) ... brakes are "hydraulic", not "pneumatic". Brakes are supposed to be a sealed system.

If I wanted the caliper metal parts to be clean, I would disassemble and clean or replace with new/rebuilt. Fluid is just a fixed mass that "pushes things around".

 

There is no circulation of fluid. You are simply pushing the old fluid out with new fluid in its place. Not only are you getting any air in the system out, but also any moisture out of the brake fluid by pushing the old out and replacing it with new.

Moisture in the brake fluid can heat up under heavy use and turn into gas form and then you can lose pressure from it.
Old fluid can have a lower boiling point and do the same.
Old fluid with a lower boiling point can also heat up so much that it can cause the rubber lines to expand a little and you can lose some pressure that way too.

In CO you can really heat up the brakes coming down the mountain passes.

I used to stunt ride motorcycles and I have cooked the rear brake fluid while doing wheelies and lose all pressure in the rear brake. Not good, when the rear brake is used to bring the wheelie back forward. A fix for it was steel braided brake lines that didn't allow expansion, and DOT 4/5 fluid with a higher boiling point.

 

well maybe the OP is asking more along the lines of this. if there a hang up or a dragging piston? you can pull the caliper and the seal, clean the inside of the caliper itself and then replace seals and look for any damage to the piston. but most of the time buying reman calipers is almost the same cost as the rebuild kit and then you can just pop the new calipers in and are good to go.

I will say this I've serviced my Toyota calipers more then any other vehicle I've owned. which is a few times. over the years, never on my Honda or Subaru that i can remember

 

Bleeding the system just pulls bubbles out of the calipers, generally, and at minimum should be performed whenever a caliper is compressed for any reason. I mostly skip bleeds and just do flushes now as it doesn't take much more effort. Flushing the fluid out cleans the lines and any moisture and contaminants out of the system. It doesn't 'clean' the pots as well as pulling them out and rebuilding the calipers will, but generally the caliper internals do not need much maintenance if you keep up on fluid changes. You're supposed to flush brake fluid every 3y/36k miles but I don't know anyone that actually changes it that frequently. I normally just do a flush whenever I change the pads/compress a piston or see that the fluid in the reservoir is dark or thick vs the clear yellow tinted color it should be (usually ~5 years here in AZ). Regular fluid maintenance also should help to keep the seals and brake system components happier. I keep a turkey baster in the garage that I use to suck out reservoir fluid before starting my flushes to try to streamline the process as much as possible. It's a pretty simple process though, just make sure you don't ever get brake fluid on your paint.

The bleeder valves should always be on the top part of the caliper/drum in order to get the air out of the system. Also I run ATE brake fluid in my vehicles.

 

Thank you for all your responses

I did find a YouTube video that explained the closed brake system in the meantime. The quick answer is, as some have alluded to, yes the new fluid should clear out the old fluid during a flush where you use a two party method. As one pumps the brake pedal the pistons and cylinders force closed to the pads and shoes. The other releases the fluid out of the bleeder for a few moments at a time. This exchanges the fluid. Then pump, release bleeder, repeat. That forces the fluid to push out old fluid that was previously pushing against the pistons and back cylinders. Assuming there are no blockages inside the caliper that would allow one piston to move while the other did not or to a lesser degree. And I learned there is no return to the master.

Others have acknowledged this too: the reason for dark fluid is several or at least two variables. Moisture and heat, but also the metals in the piping and pistons and seals are slowly chemically eroded by the fluid itself. With the biggest proportional erosion due to moisture H2O oxygen - this is just a logical guess based on my chemistry classes.

There must be eddy currents within the system that allows tiny particles to move slowly back toward the reservoir and around the entire closed system over time. It could be the particles are lower specific gravity and tend to float in the fluid.