78 300sd brake pedal going to floor after master cylinder replace and bled
 

I have a 1978 300sd with brake issues. The pedal slowly goes to the floor when the engine is running but not when off. I have replaced the master cylinder twice, both rebuilt form auto parts store, and bled the system from the brake cylinders. I have checked the vac at the pump and get 25inHg. The booster is holding vacuum also. I have looked for fluid leaks and find non. Now i am at a loss. Could air in the system cause this?

You usually need to bench bleed the m/c prior to installation to ensure there is no air. Bleeding at the wheel cylinders won't free the m/c of air if any is trapped within it (likely).

I've successfully bled it on the vehicle using a kit from Napa. Tubes are connected to the outlets of the m/c and routed back into the reservoir.

agreed. also in question is quality of rebuild... used is better than mcparts junk

Why would a used master cylinder be better? Replacing a 30 year old wear item with another 30 year old wear item means you're likely gonna be back in there replacing that sucker again in a short period of time.

I did bench bleed the m/c prior to installation.

a quality rebuild would be great, and better indeed than an unknown used MC, but the crap shoot you get from a mcparts for mc's is a guess at best.
if you bench bleed, and flushed the lines, and got all the air out of the calipers, then the only thing left is bad MC.

Thanks. Plan to try full bleed tomorrow. If that does not work another m/c i guess.