Can someone explain brake master "bench" bleeding to me? Gotta replace it

[ChrisS95TA]

Well, have my new master cylinder from Dal, came with a new reservoir which I did not expect. Anyway I know I have to get the master cylinder bench bled off of the car, after it is done would the shop just hand me a bled cylinder filled with fluid? Does the action of pumping the piston thing in there actually pump fluid into the cylinder and then seal back up without any air after bleeding? (ie - no air can get in until it is reinstalled and the brake pedal is pushed)

I ask because a shop told me by the time I got it home it would have air bubbles in it which I can see because it looks like the holes for the metal brake lines just lead right into the master but maybe I am wrong if it works as described above and seals up until the pedal is pushed again.

If someone can make this simple for me I would appreciate it, I would like to take it to a shop and pay like $20 to have it done properly, put it on my car and bleed my brakes after doing pads/rotors.

THANKS for any help, looking forward to doing this in sub freezing temps

[Benm109]

When you bench bleed a master cylinder, you're just getting rid of all the air that gets trapped when you add brake fluid to the dry unit, just like you stated. To bench bleed a master cylinder, you put the master cylinder in a vise, etc. to hold it steady, and you add brake fluid to reservior, but you also have to buy a bench bleeding kit or make some custom bent and flared brake lines that go from the two master cylinder ports up back to the reservior while staying submerged in fluid, so that way when you push in on the master cylinder piston, brake fluid flows out from the two ports on the side of master cylinder, back to the reservior, thus letting all of the air bubbles out, similar to just regularly bleeding the brakes.

Clear as mud? I wish I had a picture of diagram, that'll explain it a whole lot better than I can.

[ChrisS95TA]

When you bench bleed a master cylinder, you're just getting rid of all the air that gets trapped when you add brake fluid to the dry unit, just like you stated. To bench bleed a master cylinder, you put the master cylinder in a vise, etc. to hold it steady, and you add brake fluid to reservior, but you also have to buy a bench bleeding kit or make some custom bent and flared brake lines that go from the two master cylinder ports up back to the reservior while staying submerged in fluid, so that way when you push in on the master cylinder piston, brake fluid flows out from the two ports on the side of master cylinder, back to the reservior, thus letting all of the air bubbles out, similar to just regularly bleeding the brakes.

Clear as mud? I wish I had a picture of diagram, that'll explain it a whole lot better than I can.

That is clear as can be, thanks. That is pretty much how I understood it. Now.....after the shop bleeds the master and takes off those lines they used to get out the bubbles, wont those holes just be open to air coming in again? Or does the piston seal back up after pushing it in and out? I have the master right in front of me also links to sites of the job, it just is not clear how I can bring it to the shop, have them bleed it then bring it home.

[Joe 1320]

When you disconnect the line to re-install in the car, any air that does enter in there gets bled through the brake lines.