ok guys I need some help. My vents stop blowing air upon acceleration. I have checked the faq. I am trying to look for a new check valve like it says. I can not find one. I have been to junk yards. I can not find one online. Is there a proper name and I am not entering the name right on auto parts web sites. I am also loosing power when the a/c clutch engages. I need to get my a/c charged. The last time I went to a mechanic he told me I had a leak. He let it out of the system. It was fine before I went in. Anyway I am not running my a/c. when I do turn it on for a second I have noticed that the clutch engages often because of low freon. This is when I lose power. Have I confused everyone enough. My questions are.

1) what is the name of this black and white check valve and any ideas as to where I can find a replacement?

2) Is it common to loose some power when the a/c clutch engages or do I have another problem?

I will appreciate any help that I get. Thanks and sorry for If I confused anyone.

My turbo does not have a check valve. I have a hose that comes straight out of the intake. That hose is cracked and I will replace that. I am still curious should I have the check valve. Was the valve just taken out by previous owners.
I am also going to check the expansion tank under the bumper as suggested. I do have a leak there. The hose that connects to it is only zip tied on as with alot of the other parts I have replaced. I guess the owner before my felt you could just zip tie everthing. The question is

Should I buy the vacuum check valve and install. Did the 87 turbos have this valve. I know that only the turbos have this check valve.

Its called the vacuum check valve and you can buy it from IPD for $19.50.

http://www.ipdusa.com/ProductsCat.aspx?CategoryID=455&NodeID=779&RootID=629

I had the same problem on my 91 740 but replacing the check valve did nothing for the problem. I may have to look into the vacuum reserve tank like the other poster suggested.

My vents were doing the same thing on my 87 740 GLE B230F.
The vacuum reserve tank mounted under the front bumper was cracked. The line that connects to it comes off the intake manifold just after the throttle body. (About the same size as the brake booster line) I plugged the line as dirt was being drawn into the engine, and the vents returned to normal operation. That vacuum check valve, if you decide to get one, splices into the line a few inches from the manifold. I intend to put one on mine when I replace the tank.
Good luck