Here is the Factory Service Manual:
Mercedes-Benz Model 114/115

At that page, click on "07.1 Diesel Injection System".
Scroll through the sub-function groups inside the white box until you get to "215 Replacing Diaphragm..."

Also, there are other directions there that may be helpful.

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1968 220D, w115, /8, OM615, Automatic transmission.
My 1987 300TD wagon was sold and my 2003 W210 E320 wagon was totaled (sheds tear).