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  #1  
Old 11-08-2010, 04:51 PM
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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?
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  #2  
Old 11-08-2010, 04:59 PM
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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.
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Old 11-08-2010, 05:01 PM
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agreed. also in question is quality of rebuild... used is better than mcparts junk
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  #4  
Old 11-08-2010, 06:00 PM
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Quote:
Originally Posted by vstech View Post
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.
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  #5  
Old 11-08-2010, 06:37 PM
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I did bench bleed the m/c prior to installation.
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  #6  
Old 11-08-2010, 07:54 PM
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Quote:
Originally Posted by H-townbenzoboy View Post
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.
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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!
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  #7  
Old 11-08-2010, 07:57 PM
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Thanks. Plan to try full bleed tomorrow. If that does not work another m/c i guess.
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