PROTECTOR 59664-32F00

Good question. Not sure of the true fuction. The closest answer I have come across is as discussed in Protector in Master Cylinder

Diverter or baffle to keep the return fluid "muffled", I guess.
Same part(s) in the front brake and clutch master cylinder of my Goldwing.

jettawreck said:

Diverter or baffle to keep the return fluid "muffled", I guess.
Same part(s) in the front brake and clutch master cylinder of my Goldwing.

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Yep I agree, if it's not installed when you bleed the brakes, and pump the brake lever, brake fluid will squirt out the top of the reservoir. Don't as me how I know this 😳 because I won't admit it.

ThruST said:

Yep I agree, if it's not installed when you bleed the brakes, and pump the brake lever, brake fluid will squirt out the top of the reservoir. Don't as me how I know this 😳 because I won't admit it.

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So, it must cover up some kind of a through hole. I wonder if there is something about that little hole that is messed up on my master cylinder? All of a sudden I can't bleed the front brakes. Tried sucking with Mtyvac, pushing up with a syringe, nothing worked. I've got an OEM rebuild kit coming. Meanwhile I immersed the master cylinder in brake cleaning fluid hoping that would clean out anything that might be screwing things up.

It sounds like you may have air at the MC banjo bolt area. Normally forced fluid from the caliper end should work remove any air from the system, depending on how it was done of course.
You could try crack the MC banjo slightly, after pumping up enough brake pressure and then either hold or tie up to maintain the brake pressure, to see if this helps bleed the system of air. Remember to close the banjo as soon as any fluid starts to escape. Don't wait to look if all air has escaped. The delay could allow air back into the lines. Repeat the cycle if necessary.
Also ensure that the protector is the correct way round in the master cylinder.

Yup, it deflects that "jet" of fluid that squirts a surprising distance when you pull the brake lever. It's not super-duper necessary when the cover on the master cylinder is installed.