Good Morn

I (a relative novice) replaced brake fluid supply lines from reservoir and master cylinder with new ones on my 1970 914-4 (also, unrelated to my question as yet, rebuilt all 4 calipers using autoatlanta kit and replaced front supply hoses). First step--Left metal supply lines unconnected from calipers and used pedal to flush lines of any crud (there was quite a bit of blackness that emerged from front after decades sitting, good), but no fluid at all ever came out of back supply lines after like 40 pumps of pedal, why? I reconnected front lines to calipers; same outcome, no fluid emerging from back lines and pedal goes all he way to floor. Connected all 4 lines to calipers. Pedal still goes to floor. No fluid leaks apparent anywhere. Should i now bleed the front then see if pedal at least does not go to floor anymore?

Also, a different issue- pedal extremely sticky, does not return on its own, you have to pull it back up. WD40 all over the bushing/spring areas where pedals pivot. Did not help at all even after extensive pumping and sitting overnight with the WD40.,

Thanks! Tom

typo in my post, of course no fluid emerges anywhere when all 4 lines are connected to calipers- but hopefully you get my gist. Basically, why is fluid not emerging from rear lines when one or both are unconnected from the caliper and pedal is being pumped? If rear lines were for example somehow clogged with rust/corrosion wouldn't the pedal have some resistance rather then going all the way to the floor? There is plenty of fluid in the reservoir, this seems odd to me but as i said, I'm a rookie

Master cylinder is probably kaput (99% certain). Either bypassing seals internally or look in floor pan near pedal sled for leakage out of master cylinder.

Since you state pedal is sticking and has to be returned by hand, it’s also possible that pedal isn’t returning far enough to open the supply port to allow fluid to enter the master cylinder.

Pedal sled bushings are plastic on OEM. Bushings are worn & sticky when they are degraded. WD-40 will only make the plastic swell more and soften due to solvents in WD-40.

Rear proportioning valve could be stuck / corroded. crack open the line before the prop valve and see if you get fluid there. There is a lot of tube to pump through to get to rear brakes. Vacuum bleed or pressure bleed would be better than traditional pump and dump.

Remove, rebuild pedal sled, replace master cylinder is your likely solution.