If you wish to do the test Jim suggests, I will give you a couple hints. First don't try this on a hot engine. Take off the air filter and follow the large black vacuum supply hose from the brake booster till it connects at the back of the motor. If you are tall enough you will be able to see it. It is attached at the base of the intake plenum about 6-8 inches below the top of the intake. The line going to the modulator attaches to a small tee fitting coming off the same source. It is within an inch or so from the absolute end of the booster source hose (this is a black hose the size of a finger in diameter).

One other point to note, if the diaphram is bad enough to cause your problem the trans will be shifting distintly harder than it was previously.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician