81 300SD May need brake booster
 

So yesterday when I hit the brakes they slid down almost to the floor. I can pump a couple of times and the brake pedal holds but it will not stop unless I push down really hard. Sounds like a brake booster instead of master cylinder to me. Actually I can still drive it if I am careful so I could not do that (I think) with a bad master cylinder.

May need one. Anyone got a good one for sale?

By the way I noticed there is some brake fluid on top of the MC but when I took off the top it was not just full to the line, it was full all the way to the top. Do not think it has ever been over-full before. Does this mean anything?

Thanks.

Also how much trouble is it to install?

actually sounds like the opposite it me. a bad booster = no vacuum = no assist and really hard pedal, along with probably other vacuum related problems. i drove a VW golf with a bad master cylinder that required pumping brakes to make stops for way too long. so yes, very possible to drive around with a bad MC.

Sounds like the opposite to me, too.

Sounds like a bad master cylinder to me too.

Well, that is interesting. I did not know that you could pump up the brakes with a bad master cylinder and it would hold pressure. But that is good news. How would I confirm the diagnosis? What should I look for? I still have brakes - the pedal does not go all the way to the floor - but it takes a lot of pressure to stop. I can actually drive it some if I am careful but it is not safe.

Does anyone have a good master cylinder 1981 300SD. Is it much trouble to replace? Do you still need to do a bench bleed?

Thanks.

That's a classic indicator of a failed master cylinder. Probably at the top of the list.

OK. Now looking for a good Master Cylinder. 1981 300SD

I have a few parts cars. But zero time to pull anything.

There are two MC's listed in Peachparts. My chassis number is 005101 so I need the one through chassis number 050789 if I am reading it right.

VIN WDBC20A5BB005101

yes, early, no ABS

OK, I see one in Peachparts for $45 and one for $90. Any reason to pay double? Anything wrong with the cheaper version????

Not sure about 126 parts, but I got the cheap Metalli Made in Italy master cylinder for my W123 some time last year. So far, so good.

I agree that it's the master but don't cross the booster off the list yet. If the master has leaked into the booster (one possible failure mode) the brake fluid can eat up the diaphragm in the booster. So look carefully when you have the master cylinder off for signs of fluid in the booster.

More likely, it sounds like yours is leaking internally between the front and rear portions of the master.

Dan

I can't think of anything that could increase the fluid level in the MC reservoir. The pedal going to the floor indicates a bad MC (piston seal leak), so change that (life threatening!). The hard pedal is probably a bad booster, or lack of vacuum to it. If the MC was totally gunked up w/ rust, that could make the pedal hard, but I have never seen it.

I would change the MC first, since fairly inexpensive ($45 @ auto parts for my 300D's). If you still need to change the booster, you can leave the MC on the brake tubes and remove the booster separately. It is hardly any more work and changing the booster is expensive and fussy. Yes, critical to "bench bleed" the MC, at least 50 strokes until no more bubbles (see youtube). I prefer doing it on the car, using the pedal. You also need a good vacuum seal between MC & booster, which could be your "booster doesn't work" problem.

Brake fluid leaked into the booster on both my 300D's (leaking rear seal in MC), but didn't degrade the diaphragm. No idea how long is sat there. I changed to silicone DOT 5 fluid, which never needs changing. If using regular fluid (DOT 3, 4, 5.1), refresh your fluid every few years or you will get rust heaven throughout your brake system. Also, insure good rubber caps on the "level switch test" buttons on the reservoir top or moisture will diffuse into your fluid quickly.

Thanks guys. Great info. Going to order the cheaper one unless someone on the forum has a real one at a good price.

Thx.