Brake master cylinder diaphragm busing

[aquamantx]

if I compress the piston where i feel pressure on my finger it will return. if I remove my finger it sticks.

[DEEBS11]

Something must be backwards like the return spring or the seal. The cylinder may need to be honed. Most do. There often is a very thin washer that covers small holes drilled in the metal part of the piston. This is the check valve and must also be installed correctly.

[Radioguylogs]

I've had cases where the piston sticks when I am trying to bleed the brakes, but it was OK once I manage to get it bled.

The condition was worse if I push the piston in all the way. When it happens, I have had progress by only pumping the piston halfway in the MC repeatedly

Maybe you are OK?

[bikerphil]

I think you should be OK too, I've had them stick a little at first, now that I think about it. The rubber needs to flex a bit and break in, it is very tough rubber as you know. Once on the bike and properly bled, the piston will never exceed halfway travel anyway.

[aquamantx]

Even if the cylinder looks really clean, should it still ne honed?

[Radioguylogs]

Even if the cylinder looks really clean, should it still ne honed?

I don't usually hone it, and have no problems.

[bikerphil]

I've never honed them, maybe just a rolled up sandpaper to get the crud out. If it's clean and smooth, it is probably good to go.

[cajun31]

I have to state I have never honed one either. Generally, once you have it clean you can take a bright light and look down the bore to see if there is any pitting. I guess I have been lucky and never had that problem in the masters. The old steel caliper pistons is another story. They are almost always pitted and is why I have always just replaced them with stainless aftermarket pistons. While rebuilding the master did you check to make sure the spooge hole was not impeded?

[aquamantx]

Cleaned out both of the holes in the master. Liquid shoots right through. bore looks really clean so I guess I try again. Maybe over pushing the plunger.

[bikerphil]

Just remember, the plunger must fully retract when bleeding. Sometimes it may not, due to the pedal adjustment on the push rod. Back it off a little if necessary. After it's fully bled, you can snug it back up to have the pedal travel adjusted to your liking.