What are these for?

[Yellow Saddle]

Why would anyone be bleeding a caliper WITHOUT the pads in the caliper?
That doesn't make any sense.

Because there is risk of spilling oil or DOT fluid on the pads and it is good practice not to have the pads in there when you bleed the brakes. Further, when you bleed brakes with used pads, you have a risk of overfilling the reservoir.

 

[Heisenberg71]

I've bled many a hydraulic car and many a motorcycle braking system. I have never once bled a system after removing the pads. I have also never once spilled DOT fluid on the pads. If you are worried about spillage simply attach a hose to the bleed nipple and decant any excess fluid into a plastic bottle. However You find the fluid level you need with the pads you are using, whether old or new. Not some separate device that does not accurately reflect the actual pad width / thickness you are using.

I'm not convinced at all, but each to their own method I guess.

 

[Yellow Saddle]

I've bled many a hydraulic car and many a motorcycle braking system. I have never once bled a system after removing the pads. I have also never once spilled DOT fluid on the pads. If you are worried about spillage simply attach a hose to the bleed nipple and decant any excess fluid into a plastic bottle. However You find the fluid level you need with the pads you are using, whether old or new. Not some separate device that does not accurately reflect the actual pad width / thickness you are using.

I'm not convinced at all, but each to their own method I guess.

Well, it is not my method. It is the method of Shimano, SRAM, Magura, Hayes, Tektro and one or two boutique parts manufacturers. I didn't manufacture the little yellow gadget in the OP's photo and sneaked it into his toolkit either. Point that Capitalised finger elsewhere please.

Further, you don't understand bicycle systems it seems. With Avid, there is no reservoir that you can judge the fluid level in. Should you bleed with a worn pad and later put in a new pad, the pistons won't move in far enough and you won't get the pads in. Newer Shimano brakes don't have a visible reservoir either. How are you going to judge the correct level there if say you have to bleed with a worn pad? The plastic device is sensible and convenient. Often one has to bleed a system, especially Avid, by removing the entire line from the bike so that you can invert, shake, tap and vibrate the caliper to get the air out. When doing that you then have to make up for the missing disc's space. This can be done with a cone spanner and some duct tape, but putting a plastic block in there that stays in place by the same pin that holds the pads is ever so convenient.

 

[Cubist]

Well, it is not my method. It is the method of Shimano, SRAM, Magura, Hayes, Tektro and one or two boutique parts manufacturers. I didn't manufacture the little yellow gadget in the OP's photo and sneaked it into his toolkit either. Point that Capitalised finger elsewhere please.

Further, you don't understand bicycle systems it seems. With Avid, there is no reservoir that you can judge the fluid level in. Should you bleed with a worn pad and later put in a new pad, the pistons won't move in far enough and you won't get the pads in. Newer Shimano brakes don't have a visible reservoir either. How are you going to judge the correct level there if say you have to bleed with a worn pad? The plastic device is sensible and convenient. Often one has to bleed a system, especially Avid, by removing the entire line from the bike so that you can invert, shake, tap and vibrate the caliper to get the air out. When doing that you then have to make up for the missing disc's space. This can be done with a cone spanner and some duct tape, but putting a plastic block in there that stays in place by the same pin that holds the pads is ever so convenient.

Also, many current systems are better bled by attaching a tube to the valve on the reservoir, then introducing fluid from the caliper via the bleed nipple using a syringe. This pushes the fluid upwards, and any contaminated fluid and air exits the system from the top.

 

[Cubist]

I've bled many a hydraulic car and many a motorcycle braking system. I have never once bled a system after removing the pads. I have also never once spilled DOT fluid on the pads. If you are worried about spillage simply attach a hose to the bleed nipple and decant any excess fluid into a plastic bottle. However You find the fluid level you need with the pads you are using, whether old or new. Not some separate device that does not accurately reflect the actual pad width / thickness you are using.

I'm not convinced at all, but each to their own method I guess.

Most modern bicycle brake systems are closed. There is no air anywhere in the system as the method is to fill to capacity, purging air as you do so. Any air in the system after that is as a result of leaking seals, not wear and tear. I have the yellow blocks in my spares box, but have never used them. Shimano brakes use mineral oil, not brake fluid, and so it doesn't go off or take on air or water like DOT fluid. Much more skin and component friendly too. Any spills can just be wiped off.

 

[Heisenberg71]

Hey, every day is a school day.
I stand informed, thank you.