1998 Isuzu Rodeo Coolant Leak

Hey,

I'm trying to determine what is wrong with my Rodeo.

Last year I was driving when I noticed my temperature gauge rising so I pulled over to find out my resivoir was empty, so I added some coolant and have continued to do so since.

Coolant has been leaking from the resivoir, where I have to refill it about once every two weeks(it goes form max to min), however I don't see any signs of coolant leaks in the driveway.

I took it to my dads friend and did a compression test and he said that it appears its leaking into my engine and that I had a bad head gasket.

Then I took it to another mechanic and he did the compression test as well. I sat there watching him as he did it twice. He pumped it to 17 lbs of pressure and then it reduced to 15 lbs and stayed at 15 lbs for 10-15 minutes and then he did another test in which it did the same thing.
He examined all the hoses and there was no sign of any leak anywhere. We checked the cap of the oil and there seemed to be no bubbles, and the oil seems to have a regular color....however I just changed the oil about 200 miles ago. He claims that there is no leak and that my head gaskets are fine and that I am fine to drive it and just continue to add it.
(Also, my radiator was completely filled with coolant and showed no leaks anywhere in the radiator)

Maybe its posssibly leaking out of a crack in the resivoir when I drive...leaving no visible trace, or am I at the beginning stage of a blown head gasket?

Any advice is appreciated

Thanks

Oh its a 1998 Isuzu Rodeo with 123,000 miles