spongy brake pedal! No signs of leak, no fluid loss, bled brakes worked good for about 10 miles now goes to the floor and i need to pump the pedal to build pressure. I was wondering internal leak in the master cylinder? anyone ever replace one on 300d and links to DIY any tricks to share??

thanks

These will get you started,until one of the W123 Experts shows.

(It sure do be sound like a Bad MC,Internal leaks or not)

Also,How old are your Brake Hoses?

http://www.peachparts.com/shopforum/diesel-discussion/142292-how-do-you-attach-resevoir-master-cylinder.html#post1066676

http://www.peachparts.com/shopforum/diesel-discussion/112993-defective-new-master-cylinder-long.html#post798232

http://www.peachparts.com/shopforum/diesel-discussion/135611-i-have-tried-everything-vac-booster.html#post997851

http://www.peachparts.com/shopforum/general-information/149045-brake-system-bleeding-master-cylinder-bleeding-damage-warning.html#post1127978

__________________
'84 300SD sold
124.128

as far as i know original. they look to be in good shape they do not appear wet etc.

They collapse Internally and act as a 1 way valve.
Visually no difference.

__________________
'84 300SD sold
124.128

A spongy feel is usually air in the line. The peddle going to the floor with no leaks sounds like a bad master cylinder. Rebuilding master cylinders isn't to difficult but usually the rebuild kit is almost as expensive as a rebuilt master cylinder. I would suggest checking both prices.
I have bled a lot of brakes in my day. It is so important to not let the reservoir get empty. That just introduces air in to the system. If it worked well for ten miles after bleeding then went bad again, it might be a bubble in the master cylinder. Did the brake fluid look really dark?
I use a vacuum bleeder but the old school way works, just not as easy to do it by yourself.

__________________
William Weatherby

My 240D did what you described. It wound up being the seals in the master cylinder (if you can consider what came out seals ) . I just replaced the master cylinder the bore was rusted and would not hone out.

It was a hell of an experience trying to get the car home with the parking brake!! Especially since its a 4spd !

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Experience : what you receive 3 seconds after you really needed it !!



86 300SDL 387,000? Motor committed suicide
81 300SD 214,000 "new" 132,000 motor
83 300SD 212,000 parts car
83 300SD 147,000

91 F700 5.9 cummins 5spd eaton 298,000
66 AMC rambler American 2dr auto 108,000
95 Chevy 3/4 ton auto 160,000
03 Toyota 4runner 180,000 wifes