Ok. So to do this. Jack up the rear as far as possible and then open the master cylinder lid and just tap the brake pedal? Just want to make sure I’m understanding it correctly.
Verify you have adequate fluid in the master cylinder.
Jack up the rear until the master cylinder is AT LEAST level, preferably
down in front. If you can't jack the rear high enough, you'll have to pull the master off of the booster and push the front down--then move the piston with a blunt object (wood dowel or Philips screwdriver). DO NOT DISCONNECT THE BRAKE TUBING.
Open the master cylinder lid--or not. Your choice. If i had a helper, or I could see it from the driver's seat, I'd want to see bubbles. If I didn't have a helper, I wouldn't bother.
Tap brake pedal MORE than what's required to take up all the slack in the linkage between pedal and booster, and between booster and cylinder. You have to move the primary and secondary pistons at least a little. Going farther than that is pointless, because the piston seal covers the bypass port. It may be that the air bubble ahead of the primary piston will have to bleed out into the reservoir before the secondary piston will start to move. Then the secondary air bubble will blow into the reservoir. When there's no more air, you'll see two little "geysers" of fluid (not air) pushing up from the bypass ports when the pedal is tapped. You'll see these same "geysers" with the vehicle level and the master tilted upward--but there could still be air ahead of the ports. That's why you have to tilt the master DOWN in front to release the air.
Tap the brake pedal multiple times. If you're watching the reservoir, you'll see when there's no more air. If not, you just have to tap and hope.
There was a time--thousands of years ago, when Dinosaurs roamed the Earth--that master cylinders had their own bleeder screws, and then all this crap wasn't needed. 'Course, bleeder screws cost money, as does drilling and machining the casting to accept them, so bleeder screws on the master disappeared a long time ago.
You've never said whether you bench-bled the master cylinders before installing them. SOMETIMES bench bleeding will eliminate this problem--depends on how carefully you bench-bleed, AND how careful you are to seal the ports during the installation, AND how fast you can connect the brake tubing once the thing is bolted on. Tilting the master cylinder like I've told you, because bench-bleeding wasn't enough, is really common.
