When you installed the new pads were the rotors cut to remove any taper that would cause the pad to turn and extend thereby requiring more fluid pressure to stop the caliper piston ? I know we're splitting hairs , but there has to be an answer for this problem.
Another crazy question ,when did this brake low pedal problem start ? Was it always there ? Was something done after which the problem became apparent ?
What if----- The MC is the incorrect one and should be one with a larger internal bore to send a greater amount of fluid out with the same stroke ?
Could there be a problem with the internal parts in the booster not transferring all the motion from the brake push rod to the primary piston of the MC ? I'm pretty sure you can confirm this by just removing the two nuts that hold the MC to the booster and pulling it away enough to observe the push rod movement.
Usually you can observe this with the top off the MC and watch for that upward spurt immediately on starting to depress the brake pedal. That's quite a bypass you made, with all the extra piping changes you are probably buying brake fluid by the drum.
The figures you submitted lead me to believe the problem is before the MC.

 

The rotors are pretty new. Besides, the wheels are off so there is no rotation taking place.

I think the low pedal was a progressive thing. I didn't really notice it but when I went for state inspection they flagged it.

Well, since I have three MCs, the origonal + two new ones, I compared the origonal to one new one and the bore is the same size. Diameter and length.

Everything is pointing to the booster now. I feel resistance at the start of travel so I don't think there is much slop in the pushrod to MC. I'll try to watch for the spurt but I remember getting a big one when releasing the pedal.

The bypass is actually quite small. 3/16 tubing, maybe a fraction over an inch long. I think it has less volume than the ABS glob. The whole thing would fit in a drinking glass. But I am into my fourth gallon of fluid.

I guess the booster is the only thing before the MC.

I wish someone would go out and observe theirs and get back to me. I don't feel like tossing another hundred+ bucks at this. I'm into it about $450 so far. And if it turns out to be the booster, that may have been the only problem and all the other stuff was a waste.

Pete

 

Don't forget the big spurt could be either from the rear shoes returning or just a lot of air in the lines. Look at it this way, if you hit something because of a low brake pedal , the cost would be a lot more. Look at the brite side . Now you can stump your class members with this problem and just maybe they'll appreciate your hard work.

 

I think I will pull the booster and see what I can see. Maybe there is some worn bumper or the shaft is bending internally.

Ouch, I just realized something. Those values for movement in the spreadsheet are small by a factor of ten. The movement is more like 0.12 inches.

I'll update it later. DUH

 

With all the experience you have to offer , just think you're the guy they come to when they can't come up with an answer. That's what separates the men from the boys, the men don't give up , they just keep banging their head against the wall until the light comes on and they come up with the answer. Let's hope it lies within the booster !
I really couldn't see you stiffening the firewall. Not yet............

 

Well, one step forward, two steps back. All these tests have been done with the rear outlet of the MC plugged.

I thought it is time to find out how bad things really are so I connected the rear lines.

I can't get the left rear to stop bubbling. The right rear gave solid fluid for six pumps then one burst of air then twenty nine pumps of solid fluid. Just what I expected if the coupling was open for a few days before reattaching to the MC.

The left rear acted funny giving bubbles every few, then a few good pumps then....

Well anyway, right now the left rear gives a burst of bubbles every time the pedal is pressed. Just at the start of the press, then solid fluid. We must have done two or three hundred pumps. I even tried shutting the bleed valve before the up stroke, then applying pressure then opening the bleed valve. I get a quick burst of bubbles then continuous fluid. It has done this for, I bet, the last hundred pumps and will continue forever I feel. I went back and re-bled the rear chamber of the MC. Bled the right rear, not a single bubble. Left rear is still giving me a burst of bubbles at the start of the stroke,

Time for a stiff one.

 

Maybe I misunderstood your post, but I always closed the bleeder screw before letting the pedal come back up. Waited a few seconds for the MC to refill then built up the pressure, opened the bleeder screw with the pedal still held down and closed the screw before allowing the pedal to come back up.

 

You are right. I sometimes get a little careless if the tube is full of fluid and don't close the valve really tight.

I'm thinking, because the air comes at the start of the push, there is air leaking around the threads and getting sucked into the valve before I tighten it. Then gets blown out as the pedal starts down. I think I'll put some teflon tape on the valve.

This is actually the second valve. The old one was a bit "ratty" from rust and didn't form a tight seal with the tubing.

This job is getting me punchy.

 

I'm sure you remember, to bleed only at each wheel , unless the MC is tilted up at the rear where air can become trapped in the MC. You are bleeding at the bleeder screws right? There should be no tubing involved . I think we're rounding home plate and if you stay with bleeding only at the bleeder screws with the pedal already being held down and not allowing the pedal to come back up before you tighten the bleeder screw.

 

Right, each wheel, in the sequence called out in the manual. Furthest first, at the bleed valve. I put a piece of clear plastic tubing on the valve and run it into a container of brake fluid. I like to do that because it is easy to see bubbles in the tubing as the fluid comes out the valve.

It seems I can make my own problems. I said the old bleeder valve was "ratty" and didn't form a good seal around the tube. I bought two replacement valves ($2.09 each) and put one in to replace the ratty one. That was when I started getting a good blast of air every push. Part way through the push. I took it out and compared it to the ratty one. It had a differient shape at the inside tip. I'm thinking it didn't close well and allowed air back in when the pedal came up. I went back to the ratty one.

With the ratty one some air seeps into the tube and into the end of the bleed valve while the pedal is coming up (valve closed) and squirts out when the pedal first starts down.

Here is the thing. The girl at the parts store looked up those valves by my van model number. She reported that she had only REAR valves, no front valves. After I discovered this problem I compared what she sold me to a valve from the front caliper and the inside tip shapes were identical. So I replaced a ratty rear valve with a front valve and made more problems for myself.

I finished the bleeding getting just a puff of air when the pedal starts as described above.

Right now, with the engine off the brakes will NOT "pump up" meaning no air in the system I believe. With the engine running the pedal goes down to about 1.5 to 2 inches from the floor with all my weight on the pedal.

I will take it back for inspection tomorrow. Wish me luck.

Thanks for all the advice.

Pete