Agree... it is not retracting enough... With cover off the M.C. when ya squeeze the lever... did it squirt ya in the Eye????
It Should.. can't get that bleed hole clear/clean enough..
Sometimes... usually actually, I put a C-clamp on the Caliper, and push the piston all the way in.. Gonna make a terrible mess... because it clears fluid thru the bleed hole!! If the resulting fluid in the M.C. is Filthy... you have found the problem!!!
Edit; A syringe will pull fluid out of the M.C. before you go and force it even more full!!!!!!!

 

Master cylinder vent hole is open. I've made my tests with the lid off (and everything covered up to avoid a splash of brake fluid).
Working the master cylinder does indeed squirt you in the eye.
System flushed with Dot 4 several times. It's clean.
I did put a thin aluminum plate behind the caliper pad and tighten the caliper bolts down with the bleeder open to retract the piston. Spun free with no drag. Pumped it up and bled the system again...... drag came back.
There is an anti-rattle device behind the inboard pad. Maybe they didn't have them in '72?
Is there something in the master to check?
Can the caliper be mis-aligned due to a worn bushing in the fork leg? I would think that it would center itself.
Man I am out of ideas for this old girl and I have been playing with it for 3 weeks. It's 110 deg here so no riding anyway.

 

Dot 4????
I'm out...Only experience with Dot 5

 

DOT 5 ain't gonna make any difference on this problem, racepres.
The only place left to look is the master cylinder. Maybe it isn't retracting all the way..... I dunno. But the rotor definitely is dragging, and pushing the piston back in the caliper isn't a fix. It just goes back to dragging again as soon as the brakes are bled.

 

I agree... Don't know what to start with if the bleeder is open... Is the piston of the MC not retracting fully???
Remove the Lever... push the piston in with a Small punch.. then grab piston with needle nose and pull....any movement??
I do Not know how Dot4 reacts to other fluids... nor the Seals/orings... I have Only experience with Dot5..., But... even Water should Function I would think!!!

 

i was told yrs ago by a hd mech that the rear m cyclinder are not serviceable and a new one recomended. he was right. first i would clean the caliper piston and bore

 

Brake rotors must run true. Cannot find the specs but with a dial indicator against the rotor where the brake pads ride, within a couple thousandths of an inch as you spin the wheel around. I have witness marks on the rear rotor where I had to bang with a brass punch(below where the brake pads ride) to bring it into spec. I caught a monkey in a bike shop jumping up and down on my wheel(rotor side down)when the tire didn't seat on the rim. Don't take things for granted. It was a brand new rotor and was out of spec after said person jumped on wheel.

 

I don't have the time to do a read of the full thread right now, BUT, have you loosened the bleed screw to see if there is residual pressure?
(After pump up and release of the brake pedal).

 

Good observation... I hadn't thought of that.
Pumped up hand lever, lifted front wheel off the ground, and opened bleeder. Got a solid "weep" of fluid. Spun wheel..... and little drag. So it looks like some trapped pressure?
Ordered a rebuild kit and will do the master when it arrives. Thanks 78lowrdr.

 

Trapped Pressure is either the MC not fully retracting... or especially if still using Rubber lines, it is collapsing capturing pressure..
Crack the Line at the MC to investigate..