Hi, I have a 1981 300 SD turbo and the brake pedal wants to go down to the floor after pressing it to stop, Is it a vacuum problem or something else? I'm still able to stop but it looses holding power after a 30 seconds. thanks.........

Once it hits the floor does it return when you let your foot off the brake to apply it again or does it just stay like there is no assist? If it returns after you take your foot off your booster diaphram might have the beginnings of a tear in it and allow it to leak down as you apply it. Can you hear any noise in the car when this happens? Another thing to look at would be the master cylinder itself. once the seals on the piston go there, they allow the fluid to leak past allowing the pedal to do exactly as you have described. How is your fluid Level. Is there any sign of fluid coming out of the boost or the master cylinder mounting flange?

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2003 Dodge Ram 2500 Cummins - Georend fully built trans, Banks Big Hoss Bundle, Smarty Tuner, 5" turbo back exhaust. 500 HP and 1100 ft/lbs to the ground.
1979 Mercedes-Benz 300CD - Lost to a four car pile up on 275, Thank You Michigan Drivers.
1986 Mercedes-Benz 300D - Sold, Euro non-turbo car.

If the brake fluid is still in the reservoir, then your master cylinder is shot.

If the fluid is gone, then you have a leak in a line or in a caliper. Easy to find - just look for the wet spot.

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Thank You!
Fred
2009 ML350
2004 SL600
2004 SL500
1996 SL600
2002 SLK32
2005 CLK320 cabrio
2003 ML350
1997 C280 Sport

No the fluid level is still good..........

Once it hits the floor does it return when you let your foot off the brake to apply it again Yes it dose hold after pump or does it just stay like there is no assist? If it returns after you take your foot off your booster diaphram might have the beginnings of a tear in it and allow it to leak down as you apply it. Can you hear any noise in the car when this happens? NO Another thing to look at would be the master cylinder itself. once the seals on the piston go there, they allow the fluid to leak past allowing the pedal to do exactly as you have described. How is your fluid Level. the level is still good Is there any sign of fluid coming out of the boost or the master cylinder mounting flange? NO

There are a few seals in the master cylinder which cause internal leaking (i.e., no fluid leaves the system) and will cause the pedal to go to the floor. In my experience, the seals hold better under hard force than under soft force.

It sounds like you need a new master cylinder. It is very dangerous to drive on yours.

One last question. Do you have to pump the brakes to get the car to stop every time? Either way it is going to be one of two things. You have air in your lines and it is taking one push of the pedal to compress the air and another to actually start applying the brakes. OR Your master cylinder is leaking internally and causing the pedal to be soft, again taking more than one push to get full brake pressure.

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2003 Dodge Ram 2500 Cummins - Georend fully built trans, Banks Big Hoss Bundle, Smarty Tuner, 5" turbo back exhaust. 500 HP and 1100 ft/lbs to the ground.
1979 Mercedes-Benz 300CD - Lost to a four car pile up on 275, Thank You Michigan Drivers.
1986 Mercedes-Benz 300D - Sold, Euro non-turbo car.

Yes that is exactly what is hapening hard holds and then looses hold.

no I don't have to pump it stops at the first aply and than it looses hold.

I guess from this I better look for a new master cylinder?

I would. While it's unlikely that it will fail suddenly (i.e., no brakes), it is possible. I would fix it before driving the car any more than absolutely necessary.

Thank you so much this site and the members rock.................

Are you able to determine you don't have a vac leak?

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83 SD

84 CD