Bubbles in brake reservoir- is my master cylinder shot?

I'm continuing to carefully go through my 'new to me' 1097A2 and today was bleeding the brakes with new Dot 5 since the master cylinder reservoirs were sucked dry at the mil turn-in. The truck is still on the trailer so I don't have any idea if the brakes were working fine before I started the bleeding. The brakes did have a firm feel before I started bleeding so I don't think there was much air, if any, in the lines at first.

Starting with the rear brakes first, I was getting a decent amount old fluid coming out of the service valve with each pump but after about a half dozen pumps, the flow slowed to a trickle. I checked the reservoir to make sure it was full of fluid but I mistakenly thought the rear reservoir supplied the rear brakes so I unfortunately started sucking in air as the front reservoir level went below the strainer. After verifying the front reservoir supplied the rear brakes, I thought "no problem, I'll just fill the front reservoir back to the top and start the bleeding process again"

Anyway, I went back under the truck to continue bleeding but still only have a trickle about a dozen more pumps. I went back to look at the master cylinder while Soldier B (my young son) pumped the brakes. As you can tell in the video, when he presses on the brakes, bubbles will come up in the front reservoir. When he releases, there is a hiss or maybe even a sucking sound. If you listen carefully, you can hear the hiss when he releases.
Am I sucking air into my master cylinder from a bad seal or oring? I wonder if we cut an oring or something from the extended travel but either way I don't see any leaks but I can't believe there's supposed to that many bubbles after so much bleeding. Thoughts?

Not likely, the front is farthest away from any source of air, it would have to go through the rear piston to get to the front piston.
You would be well served to get a vacuum brake bleeder kit.
Until you get a good flow to the calipers you will never get the air out.

I'm assuming that your son is not pumping the brake pedal, but instead is pushing the pedal down and holding it down until you retighten the bleed screw at which time he let the pedal up and fluid in the reservoir is sucked back into the system and the process is repeated?

glcaines said:

I'm assuming that your son is not pumping the brake pedal, but instead is pushing the pedal down and holding it down until you retighten the bleed screw at which time he let the pedal up and fluid in the reservoir is sucked back into the system and the process is repeated?

Click to expand...

Correct. I'll open the bleeder, request (yell) for him to push, watch fluid go down the clear tubing into in the clear catch bottle, tighten the bleeder, and request (yell) for the release. He's doing a good job. Probably doing better than I did when I was his age and my dad was yelling at me from under his Ford F100....

But yes, he's not "pumping". Either way it's good to be clear and precise when servicing brakes.

Edit: Adding that the video was actually my son 'pumping' but without me underneath the truck opening the bleed valve. For the video I'm just holding my phone over the reservoir while he pumps a bit just to show the bubbles coming out.

Quick update to probably close out this thread (and maybe help others out there).

After thinking about it a bit from what Mogman said, the front reservoir bubbles from pumping were probably just the air I sucked into the lines when I let the fluid go below the strainer. I pumped the brakes about 20 times and they stiffened up pretty well and the bubbles went away. I did a quick bleed of the rears again and it seemed good. The fronts bled even easier as the bleeders are easier to access.

So long story short, once I filled the reservoir back up, it just took a few mins of slow pumps to get the air out as gravity was working the air back to the master cylinder. The hiss I kept hearing during the release was the master cylinder sucking in more fluid. The hiss was very prominent when I was bleeding the fronts and I would tell my son to release the brake.

Looking back, my biggest frustration on bleeding the brakes was trying to find enough of the Dot 5 fluid as I didn't realize it wasn't a common thing in a lot of part stores. Napa was the only store where I found quart size bottles.