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

[gottofixit]

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?

[Brian Carlton]

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.

[vstech]

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

[H-townbenzoboy]

Originally Posted by vstech

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.

[gottofixit]

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

[vstech]

Originally Posted by H-townbenzoboy

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.

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.

[gottofixit]

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