I just changed my heater core and I went ahead and changed the ac evaporator. I have leak somewhere in the ac symtem I haven't taken the time to find yet. I didn't use the ac at all this summer. Winter is coming so I figured I'd wait to spring to fix it. My question is will it hurt anything by leaving it empty with no freon while running the heater and defrost? I've heard that the ac might cycle on when the defost is on.

No. The compressor will not start unless there is a at least 35 PSI on the low side. So if the system is empty without pressure then the compressor will not run at all. If you want to insure no damage just unplug the compressor or remove the A/C relay.

 

If it was me I would recharge the system enough to circulate some UV dye. Then start doing your leak checking during the fall season with a black light. Another way to deal with it is buy a o-ring seal kit and spend about 1 hour and change all the o-rings. Some part kits come with the wrong sized o-rings and don't seal properly. (like your evaporator replacement).

better to have some pressure in the system to reduce the amount of moisture you let in. Nitrogen is a good substitute mix with a little uv dye and pull the relay like said before. find the leak and change out the accumulator with the dryer gives you a head start on moisture removal, but not till you have found the leak and repaired. then you will need to evacuated the system and charge.

I found a cheap leak detector on e-bay. Some buyers said they used it every day in their business. Less than 1/2 hour time at a garage. Also uv light but it didn't find anything.