Sounds like a pretty heavy vac leak somewhere...

If this is the case, it will upset the operation of the transmission especially the point and the quality of its shifts....

That white-thing on the IP the vac line and the Trans. Modulator are connected to is the (Transmission) Vacuum Control Valve, or VCV. Its job is basically to tell the trans the position of the accelerator, (Via linkage) and the I.P. throttle-shaft, Via Vacuum Intensity.

The trans. 'expects' to see a Vacuum of somewhere around 10 In Hg, with accelerator Not pressed, engine Idling.

The VCV is supplied with Vac, from a metered 'Orifice' which is in the main vac line from the brake-booster to Vac. pump. (The small side-barbs connected to the smaller vac. lines) The purpose of the orifice is to reduce the 20 in Hg to around 15 in. Hg

The VCV varies the vac from around 10 in Hg to Zero or thereabouts at full throttle position, This is important as the shifts will be either very harsh or will flare horribly depending on whether the Vac at a certain accel. position/engine-speed is either too high or too low.--MANY Transmissions have been replaced Needlessly due to Incorrectly diagnosed Vac. issues!

Personally, first I would check all the vac components in engine-bay--Especially the rubber joints and pipes. A small leak can drastically upset the trans. operation as the vac level will not be as intended, messing up all the vac. levels.

Check out petershmidt's site for the vac. diagrams for your car's trans system to make sure someone hasnt 'Fixed' it before and messed it all about--(Very common)!

--Its a good idea to plug the door-lock circuit to rule this out in the testing to locate leaks as others have said.

__________________
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Alastair AKA H.C.II South Wales, U.K. based member

W123, 1985 300TD Wagon, 256K,
-Most recent M.B. purchase, Cost-a-plenty, Gulps BioDiesel extravagantly, and I love it like an old dog.

W114, 1975 280E Custard Yellow,
-Great above decks needs chassis welding--Really will do it this year....