intake gasket
May be the heater core leaking
WATER PUMP IS LEAKING,.
Search for a coolant leak and take care of it before it gets worse. If you smell it, it's leaking from somewhere.
Perhaps water pump is leaking?
Heating hoses are exposed and need to be replaced. The heater core is leaking coolant.
If you have rack and pinion steering, the steering gear will need replaced.
Bad solenoid, leaking bags
It depends on where it is leaking. You can just buy a rebuilt dryer from Suncore Industries for $30.00.
There are lots of places that can leak freon and some can be sealed with stop leak. The only way is to try it.
I am assuming you are referring to a spring brake assembly on air equipped vehicles. If this is the case, normally a leaking chamber would not cause an air compressor to go bad. Some causes for air compressors not to build pressure are: defective air governor, restricted air intake and discharge hose (hose from compressor to air dryer or air tank), bad check valve at dryer or tank, leaking discharge hose (mainly the cloth covered-Aeroquip-spray soapy water at hose with engine running, if it bubbles hose is leaking thru the cover), unloader valve at compressor and some other. The ones I mentiond here are probably the most common.
If the compressor has failed internally there is really nothing a backyard mechanic can do to repair it except replacement. But leaking fittings, or a bad clutch pulley can be easily done. You will need to find out which problem you have and go from there.
The seal on the shaft coming out the front of the air conditioner compressor.