Based on your descriptions, here are the potential culprits:

When on the second button (solid arrows up and down), there are only two changes when you switch from heating to cooling mode (by the temp. dial):

1) The third vacuum switch (mounted behind the pushbutton unit, in a row of five, each with an electric plug and two vacuum hoses) actuates (opens) allowing vacuum to go to the center vent vacuum element and to the second stage of the two defroster element (the "elements" are the pink and gray plastic things with rubber diaphrams that open and close vent flaps). If when you are in cooling, you get air out of the center vent, then that vacuum element is ok...if the center nozzles dont blow air, you may have a leak in the center vent element, or the defrost element discussed below. This center vent element is toughest to get to, located way up under the middle of the dash, behind the center nozzles. Now, from the same vacuum line, vacuum goes to the second stage of the defroster element....this is located in the driver side foot well, kind of behind the ingnition switch, and has two vacuum attachments, one on the bottom, one on the side. The second stage is actuated by the side attachment....there are two rubber diaphrams in this element upper and lower...if there is a leak in the upper, defrost vents wont open, if leak is in the lower, vacuum will pass down to vacuum switch number one and leak out the bottom of it (this switch is closed, allowing vacuum to pass out the back) you may hear the leak, behind the pushbutton unit. To test the two stage element, pull off both vacuum lines, and pull a vacuum on one nipple, then the other...each should hold a vacuum with the other nipple open to the air.... if not, replace the element. Now to test the center vent element, attach your hand vacuum tester to the line coming off the side attachment of the defroster element, then pinch off the line coming from the third vacuum switch behind the pushbutton unit, and pull a vacuum. Pull and release vacuum a few times....if you hear the center vent flap open and close, and it holds a vacuum, the center vent element is ok...if not, that element is ng...real tough to replace (I think you have to remove the dash!!!).

2) The only other possible difference in the system when you go from heating to cooling is that, you may be in "recirculating" mode (fresh air flap closed) or in "fresh air" mode (fresh air vent open). The fresh air element is located far to the right under the dash behind the glove box. Remove the glove box, remove vacuum line from the element, and test the element to see if it holds vacuum.

That said, the fact that you said that you hear leaks behind the pushbutton unit, the problem may not actually be with those switches...as described above, there are conditions where a leak in a vacumm diaphram element may allow vacuum to escape out of the back of one of these switches. However, it is a good idea to pull each of these switches out, attach a hose to the nipple extending from the front (not the side one), and try suck air through it...you should not be able to get air. Then attach tube to side nipple, and suck air...you should be able to pull air through the back vent of the switch. Then repeat the above while applying 12 volts to terminals, and you should be able to suck air through the front nipple. Then attach tube to side nipple, block the front nipple, and you should not be able to pull air through the vent on back of switch.

Once you are sure these elements are working, then do tests on other system components. I have had to replace two of these vacuum switches on my 1985..with good used ones...Im getting close to the time when I may buy five new ones at about $25 each so I wont have to check them every time I have a climate control vacuum problem.

good luck,

Mark

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1984 300TD Wagon, 407,800 mi (current daily driver)
1985 300DT Sedan, 330,000 mi (gone to that great autobahn in the sky)