Low modulator pressure will cause that trans to slip. Somewhere between where you are at and slipping is a modulator pressure that gives good shifts. It will not vary from shift to shift so its only usefull for general conditions such as yours.

To do it right modulator pressure should be measured against a standard and if right and shifting is wrong, there is another issue. When testing mod pressure the value is for zero vacuum, but once that is established the value at idle vacuum should also be noted and it should be way different. If vacuum were applied to the modulator with a mitivac while the gauges were measuring mod press, a steady change in pressure should occur between zero and idle vacuum.

These cars will shift much harder if the power (and thus manifold vacuum) is reduced for any reason. Thus a smooth powerfull motor is often part of the cure.

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