Warm water and a gel shampoo .. 50/50 mix ..
Whip the mix up real good with a 1' paint brush and apply the foam to the head of the compressor..
You will see the leak as it will blow out a bubble as large as an orange..

Kids Bubble mix works well too.
They make leak spotter liquids , but I have been using shampoo for years ... I use it for pinpointing after I have located the general vicinity with my GE Halogen leak detector..