79 TD, brake pedal goes to floor - PeachParts Mercedes-Benz Forum

vstech · #1 10-19-2006, 11:29 AM

I am pretty sure the MC needs to be replaced, but just thought to run this past you guys.
driving in stop and go traffic, normal pedal, then gradually the pedal would go further and further before pressure would build up. then the pedal went all the way to the floor, but did engage what felt like the right rear caliper. I tried pumping the pedal, no go, then I hit the pedal quickly and released, this seemed to free up everything, and the pedal started working normally. I plan to pull the caliper and swap it for one off my parts car 78D (any problmes with this swap?) at that time, I will drain all brake lines and flush with Dot 3 then fill with dot 4 synthetic. I will look over all the flex lines and replace them as well... just wanted to know if any of you agree it has to be the master cyllinder.
John

DustyRusty · #2 10-19-2006, 11:41 AM

remove the cover to the MC and see if there is fluid in it. If the fluid is low, start looking for a break in the system, such as a rusted brake line. If the fluid is dark in color, then it hasn't been changed in a long time, and is contaminated. A complete flush is in order if that is the case. If you are going to switch to a synthetic, you should use DOT5.1, since it is the best you can get and will last forever. Here is a good list of brands...... Florida Approved Brake Fluid List I always like to do a complete rebuild of all components when switching over to synthetic, since it will be the last time that you will be changing fluid, and in theory, the braking system should then last almost forever....

vstech · #3 10-19-2006, 12:04 PM

and the fluid is clear. so that is not the problem, I figure the piston in the MC is old and it's not building pressure all the time. yeay me.
John

t walgamuth · #4 10-19-2006, 06:52 PM

yes, but look in the res and be sure the back section is full too. if it is empty then as noted above, look for the place the fluid is getting out.

if both sections are full of fluid then it is time for a new master cylinder.

good luck

tom w

Ara T. · #5 10-19-2006, 08:51 PM

If the master cylinder is close to 30 years old, obviously replace it. Braking will improve exponentially. Before you do that though check all the metal brake line unions to make sure they are screwed in tight, I've seen some cars where they vibrate loose. Check the lines to the MC and to the calipers. If they are all tight then it is all due to MC failing.

t walgamuth · #6 10-19-2006, 09:12 PM

well it could be leaking anywhere in the system. lines going to the wheels rubber lines or calipers. so check it all.

tom w

mbzkid · #7 10-19-2006, 10:49 PM

Use DOT 3 or what is reccomended by Mercedes Benz. You can't just pour in a fluid becuase it has a higher DOT number.

babyjames · #8 10-19-2006, 11:14 PM

The wagons have a different, unique rear caliper. They are quite expensive, too. The rebuild kits are pretty cheap, however. Good luck! It is the rear caliper you were asking about, correct?

Jay.

t walgamuth · #9 10-19-2006, 11:29 PM

Quote:

Originally Posted by babyjames

The wagons have a different, unique rear caliper. They are quite expensive, too. The rebuild kits are pretty cheap, however. Good luck! It is the rear caliper you were asking about, correct?

Jay.

and the way they are different is the bore is a little larger. the pads are the same as on a sedan though.

tom w