My wastegate is not holding pressure when I pump it up. I can't seem to find one for sale. What can be substituted?

The 1990-1993 300D uses a vacuum actuated wastegate that is controlled by the EDS. This system tends to become unreliable with age and is expensive to repair. You can replace the vacuum actuated wastegate with a pressure actuated one. Many others have done this with great results, including myself.

Here are some threads about it:
Wastegate actuator swap for OM602.962?

Need Help 1992 300D Turbo Failure

Also, this should be posted in the Diesel Discussion.

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1998 E320 Wagon

1992 300D 2.5 TURBO

1991 560SEL M119.960

1986 300SDL TURBO

1992 500SL Parts Car

1991 420SEL Parts Car

As mentioned, your best bet is to convert to a pressure actuator. But if you are interested in a working vacuum actuator, let me know.

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 154k miles
06 E320 CDI "Rutherford", Black on Tan, 172k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 142k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 61k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife