I'm trying to bench bleed an NOS disc brake master cylinder using DOT 5 fluid. I'm using a NAPA bleeder kit with brass fittings and hoses from fittings in outlets into the reservoirs. The secondary bled quickly, but the primary (outlet closest to flange), I'm still getting bubbles.
Been at it an hour. I tightened and retightened the fittings. Still keep pumping away with no luck.
Any ideas, I've just about given up that I'll ever stop getting bubbles out of the primary outlet?

Are they entrained bubbles?...try changing out the fluid

I've heard issues before with it hard getting all the air out when bleeding brakes using DOT5. I've only used it once before, when I changed out a bunch of components like calipers and MC in my brakes, and switched over to 5 from 3 at the same time, and don't recall any issues. That was in the 90s though.
I used fluid that had been sitting, didn't jostle it around or shake it before pouring into the reservoir. I've heard people letting the container sit in the sun before using, I didn't do that.
I just now tried putting plugs into the fittings instead of the fittings with the lines. Pushed the pistons forward and got a few little bubbles rising from the fill port in the primary reservoir the first time or two I pushed, then no more bubbles.
Got fluid shooting out of the fill port in the secondary.
I could only push the pistons about a 1/4 inch forward with the outlets plugged.
Been doing this outside, getting dark, and I may call it a day with this. Let it sit and see if I get better results tomorrow evening after letting it rest.

Try getting a child's nose sucker ball or similar fill with brake fluid & connect too the hose & back bleed the master,dont squeeze too hard or fluid will splash out,angle the master so the hole in the bottom is on the upside & watch for the bubbles to clear up,I have an old EIS master back bleeding syringe type thing that works great.

I have used dot 5 in everything I could, heck even my 97 Harley came with it stock... Never had troubles with it like I have read on this site....

Not criticizing you, just saying it oughta be OK.....

Best way I've found to bench bleed a master is to just hold my fingers over the ports and push on the piston, let a little fluid fly and hold my finger back on the port before I release the piston.

Its a little messy, but it works great and only takes a matter of seconds to fully bleed the master.

the neverending stream of bubbles would have to be coming either from the threaded fitting (you might daub it with something to see if it gets sucked in) or from the rear seal

I'd think if the rear seal is bad it will ooze fluid as it sits. If the flange is dry tomorrow below the bore, that will be a positive sign.
It's an NOS cylinder, I was worried about the seals, but had pulled the primary piston assy out to inspect, and the 2 seals seemed very pliable, so I am optimistic they're good.

I ain't an engineer but I would think a vacuum could pull air in past the seals much easier than fairly viscous fluid could seep past em but yes I would more suspect the threaded fitting(s). post what it ends up being

Just because the master cylinder is NOS doesn't mean it's going to work after 45 years or so of sitting around. Using the older seals that were meant for dot 3 or 4 doesn't mean they are going to work for the newer fluid.

I used to work for an aftermarket brake parts manufacturer, and EPDM rubber seals are compatible with both conventional and silicon brake fluid.
Just don't let them near any petroleum based fluids!
We made brake cylinders for government applications like military and USPS and they used DOT 5, we used the same EPDM seals for those products.
But yes, the fact these seals are old concerned me, but I disassembled the MC before adding fluid, and the seals were soft and pliable, no oxidation or stiffening from aging that I would be worried I might find.
So I decided I trusted these vintage Bendix seals more than rebuilding with new Made in China seals.
But it is still possible the seals have a problem being decades old.
The only seal that could possibly allow any air to enter would be the one by the bore on the primary piston, and that is a pretty wide surface area seal. The cups on the front of the pistons are usually the ones that are first to go bad, as they are smaller and build pressure when brake pedal pressure is applied.
I found this article on DOT5 which is interesting, especially on the subject of air entrapment with it. I wasn't especially gentle dumping the fluid into the reservoirs, so maybe that caused problems? Though the secondary stopped getting any air quickly.
http://www.mossmotors.com/SiteGraphics/Pages/Brake_Fluid/page4.html

Following up on my MC bleeding issue. I called it a day last week after posting, and between work and working on a number of other things on my project including installing the exhaust last weekend, didn't look at the MC again until today.
I removed the plugs in the connectors I had in the outlets, and installed the clear lines from the NAPA bleeding kit, and slowly pushed the piston in a few times until fluid started coming out. Other than a few small bubbles in the secondary outlet, no air!
Letting it sit dispersed the entrapped air or something I don't know.
But at least the master cylinder seems bled now.
Now I have to worry about charging and bleeding my virgin brake system. I've started gravity bleeding but so far little or no fluid seems to be draining from the reservoirs. Not sure what to expect here, I've never done a system from scratch. Replaced Calipers many times, other times wheel cylinders, lines etc, never had trouble, so hoping this works out for me!

The fluid in the reservior need time to sit. This allows bubbles to float to the surface before it gets pushed down into the system.

Gravity bleed is ok for maintenance but I'd do the traditional pump, push, open bleeder method for a new system.

As said above I'd either regular bleed or at least several strokes to get the fluid flowing first. then a gravity bleed as needed. Calipers might need "regular" bleeding to get a big chunk of air out of there. I have a helper pump the pedal vigorously several times then hold it down & with perfect timing as he is on the 3rd downstroke I crack the bleeder which greatly helps to get the agitated air/fluid mixture out of there. wheel cyls might not need that turbulence.

When I did my 70 Sport Satellite, I started from scratch too - everything replaced. Anyhow, I used a hand vacuum pump to suck the fluid to the wheel cylinders/calipers . . . made sure to keep the mc full. Once everything had fluid to it, then I had a buddy help out and do the old pump, hold, release . . . . good luck . . .

Many years ago I built a jig and used a air cylinder, air piloted valve and threshold nots to build a self bleeding pneumaticaly actuated bench bleeder.

You could run it for 24-48 hours and still get very small bubbles. The same amount as after initially bench bleeding it by hand.

I bolted the mc on and bled the rest of the system in the conventional way and though I ended up with a extremely firm pedal wasted a ton of time and a little money.

When I described the system and process my friend who is a mechanic shook his head bolted a mc on a customer car filled it and bled the whole system. He had a firm pedal too in about 10 minutes.

Enjoy!

Red