I don't see how but maybe the line is blocked. If the line was blocked there would be no vacuum leak.

Also perhaps the transmission vacuum valve is no good any more.

On my 84 300D there is an plastic orifice stuck in the vacuum hoses before it gets to the Transmission Vacuum Valve.

When I had no Vacuum to the Transmission Modulator (I had disconnected the line with out knowing it) the up shifts were nearly the same but the down shifts were sudden and harsh.
Also I don't recall the disconnected line causing any breaking issues. Between the orifice and the Transmission Vacuum Valve which is a variable orifice I guess there is not enough vacuum loss to effect the breaks.

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84 300D, 82 Volvo 244Gl Diesel