1991 560 SEL Vacuum Check Valve for AC

I have a vacuum problem with the AC in that there is air coming out the defroster vent at same time as dash vents in the normal AC mode going up hill ( decreased vacuum ) it gets worse. There is a yellow vacuum check
valve on the wall that is in front of the fuse box but slightly toward the middle. It has one input and 2 outputs. Great vacuum coming in. Have not checked the out vacuum as the lines seem to be fused to the plastic valve. What is the chance that this valve could be defective as opposed to a bad vacuum motor. If you disconnect the vacuum line then the air all goes to the defroster vents and side dash vents (default position I guess) Thanks Barry
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Well i bypassed the vacuum check valve and sent the vacuum directly to the tube to the AC and it works fine except without the valve it does not check on hills etc. So out to Mercedes to see if the valve that they have is the right one. Only about 9.00 - surprise.
Al wrote:

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Well Al, you were right on the spot before I could even answer your question... that is absolutely correct... yellow checkvalve is at fault when you lose vacuum going uphill or full throttle.
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