AC and Vent questions

I have a few general questions about my ac. A year ago in FL I paid a bunch for switching my ac to 134a. Got a new compressor, liquid line, evaporator. Everything worked fine, then in the winter I replaced my heater core. While I was at it I replaced the fan motor. In the spring I recharged my ac, but didnt really use it much.
Now, my ac was cold for awhile, but its completely not cold anymore. I just bought some die and weather permitting im going to look for the leak today. Any idea if I might have done something when I did my heater core? To cause a leak I mean. I had to disconnect the ac as im sure you know to do the heater core.
Anyway, my next question is about me blower motor. Its been so long since I used it I cant even remember. I am a little worried maybe we switched a vaccum line? The fan works fine, but when I put the switch to MAX AC, a vent opens under the glove box, and its very loud. It also seems that (when the ac was cold when we first recharged it) that it wasnt coming out as hard or cold on max ac. Is that vent supposed to open for MAX AC?
I was wondering if it was supposed to open for VENT (circulating the interior air?). There is no change in the blower when I switch from vent to AC. Any idea if thats just working right or if I have something backwards?
Sorry for the length, want to try and give all the information I can. Hope this helps, and thanks in advance for any help.

Thanks a lot for the information.
Well, I put in red leak detect dye, and havent seen anything yet... I filled it back up with refrigerant, and the ac is running fine at the moment. I had another question however.
Why does the compressor clutch engage and disengage so often? Is that normal? My girlfriends 95 Sable and fathers 99 escort do the same, but not so frequently as mine. Is that relevant to the Low Pressure Cycling Switch mentioned?
Further, from the gauges, when engaged I get a a best reading of about 27 on the low, and 170 on the high. Yesterday it was raining and not very hot. I thought that was a little low on the high side pressure, but like I said it wasnt very hot out. What should I be seeing? Those numbers are the numbers at maximum length of the clutch engaged.
Thanks for the help and information.