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1978 300sd
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Discussion Starter #1
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 the checked the vac pull of the vac 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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1987 420SEL
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1,133 Posts
You have to bleed the master cylinder itself, and essentially bleed the top of the system.

Then you can bleed the lines from the passenger rear wheel, driver's rear wheel and so on until you get to the wheel closest to the master.

I'm not sure how to bleed the master cylinder itself on your car, but that is what needs to be done. If there's air in it, no amount of wheel cylinder bleeding will get it out.

Power bleeder?
 

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1987 420SEL
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1,133 Posts
In that case, to answer the original question of whether or not air in the system could cause the power booster to draw the pedal down- yes.

Check all flare connections for leaks, check all lines for trace amounts of fluid.

There's either air in the lines or something is damaged.
 

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1965 220S, 1999 Volvo V70 (wagon), 2006 Ford Crown Vic Police Interceptor, 72 350SL 4 Speed
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1,830 Posts
You don't need another master cyl. you need to bench bleed it, by using two short tubes from the pipe connections to the reservoir. Pump it by hand until the fluid is clear and has no air in it, then install it on the booster and connect the pipes.

How are you bleeding the system? You must first bleed the farthest one, the rear passenger side, the rear drivers side, then the front passenger side then the front drivers side, assuming LHD.

Get a power bleeder, such as Motive Power Produccts pressure bleeder or one that attached to the front tire and screws onto the reservoir.

Always use new brake fluid and wear saftey glaasses. Brake fluid is very bad for your eyes.

At this time it would also be a good idea to replace all the brake hoses. There is also a proportioning valve on the drivers side in front of the rear axle.
 
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