How do I tell if my Master Cylinder is Bad???
 

I have a new w124 300d (1987) and the brakes are not good at all on it. How do I diagnose?

How are they not good? Have you inspected the pads and rotors? And the rubber hoses connecting to the brake calipers? If all that is in good shape, then check the brake booster (if the pedal is hard to press it may be bad, or may not be getting vacuum) Only then should you suspect the master cylinder.

The brake pads still have some of the brake material on them. I'll be changing those soon but I'm noticing I've got to pump the brakes to get them to work good.

Could be the hoses, check (and replace them if old, its cheap preventative maintenance), old hoses can "swell" when pressure is applied to them, thus reducing braking power and making the pedal feel soft or spongy....

After you pump up the brakes, keep a lot of pressure on the pedal and see if it holds.
If the pedal slowly goes to the floor you probably have a leak either in the hoses or the brake lines, possibly a caliper.

Abrupt deceleration, loud crashing sound and airbags deploying are usually telltale signs...:rolleyes:

Seriously, check out everything else first as the other posts suggest. Pads, hoses, calipers, etc and ALSO drain & replace the brake fluid with the OEM recommendation(-check the manual for the right stuff). New fresh fluid helps to lengthen the service life of many of your brake components and improves their performance. It is cheap and well worth doing. Hoses are relatively inexpensive too and usually get overlooked for way too long.

If you need to pump the brakes sometimes, but it holds, and the pedal doesn't sink to the floor, what would you think was wrong?

On the W123 cars (trust me I know first hand) and I believe many Mercedes from that era, the failure mode for the Master Cylinder locks pressure on the rear brake circuit. I think they designed it to fail in this manner rather than losing brake pressue, for safety, especially on roads like the Autobahn. My master cylinder failed in town and I thought both rear calipers froze. I replaced the calipers but the problem still appeared after the brake fluid was warmed during normal driving. Letting the system cool allowed the pressure to relieve itself, and I was able to drive a short distance till the system heated up again. I replaced the master cylinder (thanks MercedesShop) and haven't had any problems since.

Clamp the brake hoses and try again. This will test the master cylinder, rather than the calipers.

I would guess trapped air in the lines, not an auto expert, just stating from my own experiences.
One bad master cylinder in 40 years, pedal was extremely hard to press down and went within an inch of the floorboard.

My master cylinder's symptoms were weak braking (unable to lock up tires) and still kind of a mushy pedal after bleeding the brakes several times. Took off the MC and noticed it was obviously leaking into the brake booster.

Paid 100 bucks for one, replaced, and the brakes are exponentially better.

Then there's the simple test that isolates the MC unit: Apply steady pressure to the brake pedal with the car standing still and engine on. How long does it take to bottom out with pedal on the floor? Perform this test often and you will track condition of Master Cylinder thats slowly dying. MC going bad can be nursed for more than a year. Yours probly started dying more than 6 months ago.

Then flush out the brake fluid *when* you have replacement hoses and new/refurb master cylinder standing by for installation and NOT before. Otherwise if you flush the system first then fresh fluid will literally pour right past worn out MC seals creating immediate brake system failure.

Here is one indication.My brake fluid level usually gets low as per dash light indicator. No leaks from the outside. When I brought it to the mechanic, my brake master cylinder was leaking. Had it replaced.