Can I Avoid MC Bench Bleed
 

I just totally rebuilt the brakes on my 83 SC. I replaced the Master Cylinder. I didn't bench bleed the Master Cylinder. Everything is back on and I've bled the brake lines with the Motive system. I'm getting no air from the lines. My problem is that I think there is air in the MC. When I pump the brakes, it seems that brake fluid is going back into the Master Cylinder reservoir. I'm getting no pedal. I really don't want to take off the MC to bench bleed. Any advice?

Subscribed. I also have a new MC (23mm) that I need to install

Get an assistant and tap the MC lightly with a wood mallet or block of wood while bleeding. It can help encourage air bubbles to migrate and escape

You could gently open each line as well. I'd wrap it in a rag to prevent the fluid from sailing across the garage.

Should I open the lines (very slowly of course) with the pedal down?

You can. You're essentially doing the same as a bench bleed but in the car. Careful of making a mess.

Start with tapping the master cylinder while working the brakes.

Sometimes the MC is just defective.

Spent days trying to get a firm pedal after mc replacement, tried pressure bleeding, vacuum bleeding, forcing fluid from calipers to mc, gravity bleeding and finally in desperation got wife to help with old fashioned two person bleed and bingo, all is well.
Good luck

I can’t figure out why I’m not getting any more air bubbles. Were you getting clear fluid but no pedal?

Bubbles get trapped in nooks and crannies sometimes. Increased possibility when you're filling a brake system for the first time. Hence the suggestion to lightly tap the master or calipers to encourage the trapped bubbles to move.

I've had great success with gravity bleeding the 911 brakes. Even after a master cylinder change a 10 minute gravity bleed was all that was needed. I do tap the MC and calipers with a metal object to encourage any bubbles to flow out.

When I pump the pedal, the brake fluid goes back into the reservoir? It's just like MC is pushing the fluid back in rather than in the brake lines?

I just replaced my entire braking system. I did not do a bench bleed. I gravity bled first (works like a charm) and then followed with a traditional two person brake pedal pump method. Maybe try to do these two methods and see how things are. If you’re getting fluid pushed back into the reservoir then as JW suggests your MC might be kaput.

I always thought the purpose of bench bleeding was to drive the piston down to the bottom of the bore several times to get all the air out. That's hard to do mounted in the car and attached to the pedal mechanism. But not impossible.

You might fashion a piece of bundy tube with a single flare, attach to the M/C, and bend it up & around to submerge in the reservoir. Since there's no resistance, you might be able to bottom the piston attached to the pedal. Four of five pumps ought to do it.

The benefit, besides bleeding the M/C, is that you'll possess a home-made tool for bench bleeding M/C's for the rest of your life.

The fluid is following the path of least resistance. When I prime an empty brake system, my initial steps are 1. fill the reservoir with brake fluid 2. crack a bleeder at one corner 3. just let it flow for a bit until there is fluid coming out from the bleeder on its own via gravity (no pumping yet).

If you start pumping too soon, you'll just be pumping air.

The reason that the fluid goes back to the reservoir is because the compensating ports are open. That's a good thing. If they were closed, when the brakes got up to operating temperature, they'd stay on, as the fluid expanding due to heat would be trapped and the calipers would stay applied.

As suggested, try gravity bleeding. And start bleeding at the MC, with someone holding down the pedal while you turn the wrench at the MC fittings.