This is on my 1980 240D...

Pedal has been squishy for a while, then the clutch started to get short (pedal had to be almost all the way to catch). I can pump the brakes and get some pressure back in the clutch for awhile

Recently the brake warning light would come on if the brake pedal was pressed and held all the way down. Now the light is on more frequently.

So I figure I need new pads, master cylinder, and likely rotors.
But how do I know if I need a new booster, too? If I am going to replace it, now would seem to be the time!

What is a diagnostic for the booster?

__________________

Current Mercedes
1979 maple yellow 240D 4-speed

Gone and fondly remembered:
1980 orient red 240D 4-speed

Gone and NOT fondly remembered:
1982 Chna Blue 300TD

Other car in the stable:
2013 VW Jetta Sportwagen TDI / 6-speed MT

Just because you mention the "clutch" as an issue, your first step should be to check the fluid level in the rear portion of the brake reservoir. The reservoir has a front portion and a rear portion, the fill opening is located over the front portion and in order to fill the entire reservoir the front portion needs to be overflowed to fill the rear portion. The rear portion providsd fluid to both the rear brakes and the clutch. If there is any leaking in either the rear brake circuits or the clutch circuit the loss of fluid in rear portion of reservoir will effect both systems as you're describing them. Sounds like you've may have lost fluid and as aresult gotten air in the rear brake section of the brake master cylinder and the clutch master as well. If the fluid level for those systems is low you'll need to find and fix the leak responsible for the fluid loss then refill the reservoir and bleed the rear brakes and clutch systems. If you don't have a power brake bleeder you should do a search of "clutch bleeding" and find the thread on using a old school pump oil can filled with brake fluid to reverse bleed the clutch and brakes from their respective bleed screws back towards the reservoir. That is a pretty straight forward, dependable and inexpensive method of accomplishing bleeding of the hydraulic of these systems.

Yeah, you're going to need to get the vehicle up and do a complete brake inspection.

If you see no leaking around the Master Cylinder, check for leakage at the calipers. Should check the clutch cylinders as well for leakage.

Could be leaking into Brake Booster. Check the hard lines for damage too.

That fluid is going somewhere! Once you find the leak, you've likely found the faulty part.

Replace brake parts in pairs - both front rotors and/or calipers and rubber hoses to maintain balance.

__________________
Codifex
1981 240D ChinaBlue (Got her running with a donor engine.)
1983 300DTurbo w/sunroof.
1984 300TD manual sunroof. (Electrical Gremlins)

I replaced the pads, and the warning light went away. I saw no fluid leaks anywhere in the brake system, and the resevior seemed to have the same amount of fluid as when I topped it off about 2 mos ago. With the new pads, the brakes felt much better, but the pedal still falls just a bit when the car is started. BUT, the clutch problem got MUCH worse!

After replacing the pads, we went on a trip to Houston (~3.5 hrs). Halfway there we stopped at a convenience store, and as I exited the highway I found no clutch at all!

Got to the intersection off the exit, and was able to pump the clutch and get enought pressure to get into 2nd which got us to the store.

At the store I pumped a lot and had enough pressure to get back on the highway. Got to Houston and had some white knuckle stop-and-go traffin, but I made it.

We were visiting my uncle and cousin who are able mechanics. Cousin helped me look over the system. We saw a bit of wetness where the shift linkages meet the transmission, and I noticed that the top of the speedo cable was oily, but the lines and slave were dry. (Also, no fluid in the cabin under the pedal).

The bleed helped a bit, but by the time we got to our hotel that night (~45 min on highway) , the clutch pressure was low again.

I found a thread here that linked to a video showing reverse bleeding by filling from the bleeder nipple on the clutch. Thought maybe we had just not gotten all the air out of the master clutch cylinder.

So we tried to reverse fill/bleed twice when we got back home the next day, and the day after. After the 2nd bleed, everything felt really solid. Clutch felt bettter than it had in years, maybe since I bought the car!

I was feeling really good until today when it failed again.

Still no sign of leaking, but the moist speedo cable bothers me. It is right under the junction where the hard line from the master connects with the line to the slave, and looking at it more closely tonight looks moist.

Is that just an o-ring in that junction? It seems like the only 2 possibilities are a leak from the slave into the transmission, or that junction.

The weird thing is that driving around town in stop and go traffic the clutch doesn't seem to suffer, but 10-15 min of highway driving and the clutch (but not the brakes) loses pressure.

Thoughts?

__________________

Current Mercedes
1979 maple yellow 240D 4-speed

Gone and fondly remembered:
1980 orient red 240D 4-speed

Gone and NOT fondly remembered:
1982 Chna Blue 300TD

Other car in the stable:
2013 VW Jetta Sportwagen TDI / 6-speed MT