hhmm.....I'm not familiar with the roadster m/c. I just looked at datsunparts.com's site and it shows a dual circuit m/c with the rear circuit toward the radiator. Like you, I've been doing them at least 30 years and they can still kick my butt sometimes. I'll run through a couple of things that you probably already know, so if anything, it may be info for someone with less experience. I'm certainly not a pro at this, so this is just my 2 cents......but it may only be worth 1....if anything
Let's see.....first off......air in the lines is typically a spongy pedal. Too much travel is loose adjustment. On a dual circuit system, you should at least feel some pressure from at least one circuit if it's good. They're designed to stop the car if one circuit fails completely. So, if the pedal hits the floor, your rear brakes are bad or too loose.
You may want to set the e-brake to eliminate any of the travel on the rear brakes. It could be that when the first circuit(front, near the firewall) collapses as you push the pedal, some of the pressure is absorbed by the second circuit(rear), so your pedal hits the floor before you feel any resistance. If your rear brakes are on the loose side, they'll have more movement. By setting the ebrake, the wheel cylinder will pump up with the first stroke or two and stay that way since the ebrake is holding the shoes in contact with the drums.
You may need to bench bleed the m/c....which can be done in the car. The point of bench bleeding is to get the air out of the m/c...obviously.....but the pedal may not bottom out the piston in the m/c. You'll want the m/c to be close to level. You can jack up the front or rear of the car to get it there. You can disconnect the linkage to the pedal and use a long screwdriver or bar.....you'll need a second person. Crack the brake line or bleed screw open on the m/c circuit you're working on and slowly push the piston all the way in. When the piston stops, tighten the brake line or bleeder. Then release the piston. Do this 2-3x's. You might as well do both circuits while you're at it. Also...while you're under the dash, make sure that you only have a small amount of play in the linkage to the m/c. It should not be pushing on the piston....there should be a tiny bit of wiggle. An 1/8" of linkage travel before contacting the piston is way too much slop.
You can hook up the linkage and bleed the rest with the pedal. When pumping the pedal, about 2 strokes per second is what I do. The pedal needs to travel to both limits.....all the way down, either to resistance or the floor, then all the way up. If it is not allowed all the way up, the piston can't refill with fluid. You don't want to pause too long at the top of the pedal travel or the fluid can back fill into the reservoir. This is another reason I set the ebrake.....I don't have to fight the springs pushing the fluid back when I'm doing the rear circuit. With calipers, the only "spring" pushing back is the rubber dust seal around the piston...which isn't much.
Everyone knows when bleeding brakes, you start with the corner farthest from the m/c. But....technically, it's the longest line from the m/c. So....if the brake lines on a roadster run from the m/c, to the passenger side and then to the rear, the longest line will be the driver's rear. If the rear line runs from the m/c to the rear down the driver's side....it's the passenger rear with the longest line. Once you're gone around the car 3-5x's and you're not getting any air, I don't think it really matters at that point, but it's a good habit to start with the longest line.
I've used my finger as a speed bleeder to get a circuit started. I crack the bleeder valve, put the tip of my finger over it firmly, then have the helper pump the pedal. The air/fluid will push past your finger, but it will seal back up enough to keep air from going back in. You don't want the bleeder valve too loose, or air can get pulled back in around the threads.
I have a simple, home made pressure bleeder that I can use to get the fluid into the lines initially. I have a bug sprayer on a shelf in the shed just waiting to make a bigger/better one with....it's been there for probably 4 years already. lol
Either I'm having a brain fart, or that's about it.
