I am ready to bench bleed a new 72 dart iron master cylinder.
I am a brake novice.
The pictures of bench-bleeding I've seen show tubes that loop back into the reservoirs so the fluid is recycled.

But this master cylinder came with threaded plugs.
The instructions say "install plugs to seal the ports"
and the illustration shows a MC in a vise with plugs, no tubes.

I'm confused.
Is this an alternate way to bleed some master cylinders?
How does it work?
Will the plastic plugs pop out under pressure and spray the kitchen w/ brake fluid?

Yep! Run down to your local auto parts store and pick up a master cylinder bench-bleeding kit. Should cost about $8. Comes with an assortment of plastic thread-in nipples and a couple short lengths of hosepipe. Thread the appropriate nipples into the master, run the hosepipes from the nipples into the reservoir, fill the reservoir with fluid, stroke the master cylinder completely a few times (until bubbles stop coming from the hosepipes) and you're all done.
That sometimes sorta works almost well enough.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

The idea behind the plugs is it forces air to bubble back into the reservoir. Problem is those plugs are plastic and don't hold much pressure as their threads strip easily when you tighten them down. And, as the air bleeds out, it gets harder and harder to push the piston which may result in an incomplete bleed.

Start off with small strokes of the piston to get some fluid in the passage. Stroking (hhuhhhuhuhuh...) a master cylinder dry is no different then running the engine w/o oil.

I didn't know to bench bleed the MC when we installed a new one on my son's car. When we finally got his car finished enough to get it on the road for a test drive, the brakes were very spongy and the light stayed on constantly. While the car was in the shop for a front end alignment, I had them go through the entire brake system to check our work and see about the cause for the constant brake light. When I picked up the car, they had finally bled the system enough for the light to finally go out. They did not take the MC off the car to do the bench bleed, but I now know it's a lot better to do that next time.

Jerry

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There's a difference between ignorance and stupidity.
Ignorance is not knowing any better.
Stupidity is knowing, yet doing it anyway.

When I did mine, I was too weak to bench bleed the MC on a vice...too hard to push it in that way. So, I mounted the master cylinder in the car with the bleeding tubes, etc. so that I could use the mechanical advantage of the brake pedal to do the bleeding.

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'68 Dodge Dart 4dr Sedan

The last couple MC's I bought (I think the last came from NAPA) came with a nice little bleed kit in the box.

I re-did the metal lines on the front of my 73 when I converted to disks last month. I just stuck it on the car and let it gravity bleed for a little while, then blead them normally, took longer then usual to get all the air out, but it worked.

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70 Dart, 360, 727

"Bench-bleeding" by mounting the MC on the firewall and pumping the brake pedal probably doesn't run the piston through a full stroke.

Is it important that the piston go through its full stroke during bench-bleeding?

I have used the little plastic plug method and it does'nt work that well. Plugs blow out, leak. Hoses work best.

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82 D150-225/727
02 Dakota-3.9/5 speed
87 GMC C7000-8.2 Detroit Diesel/5+2

But when you bench-bleed a master cylinder, are you supposed to push the piston in as far as it will go?
Are you supposed to use complete strokes - full range of travel - to do the bleeding?
If so, is "bench-bleeding" with it mounted on the firewall, using the correct tubes and everything, inferior to doing it on a bench with full strokes?

Or is it unnecessary to push in until it bottoms-out each stroke?
Will it damage a seal or something?

I don't think it's super-critical that you run the master through absolutely complete strokes. "Most of the way" should be fine. But, if you do use full strokes, it shouldn't damage any seals.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

I put it on a bench and did the full stroke thing.

While I was bleeding the RR I let the front reservoir go dry.
Now I have to bench bleed again, right?

Yep! This is why it's easiest to run the bench bleeding tubes from the master cylinder outlets back into the reservoirs.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

I did run the tubes that way during bench bleeding.
The bench bleeding step was successful.

I let the reservoir level get too low while bleeding the rr wheel.

I am going to take a break.

Don't you just hate that..........

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Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes