We purchased a reconditioned unit to replace an older unit in a mobile home. The unit worked in the shop where we purchased it. We hooked up the 2 110V lines and then turned on the thermostat to cool. Immediately following, the main 100 amp breaker popped. We then disconnected the thermostat wires and pushed in the relay manually and it popped the main breaker again. Is it something we wired wrong or is it in unit?

The main breaker may be defective or have a loose connection, or there may be a miswire, without knowing any amps or seeing the wiring , hard to be specific.

When you say 2-110 lines, you mean using a separate 2 pole breaker just for the AC, correct?

Yes, we are using a separate 2 pole breaker. We used 2 8 gauge wires out of the bottom of the double pole breaker. Then hooked up the wires to the bottom of the power block. One on each side. One side has a relay switch the other does not. If there is a problem with any breaker, how do we test it? There is a 60 amp in the a/c disconnect box and 2 70 amp breakers in the main panel in the house.

The wiring sounds OK, is the power block in the AC unit? The relay should be for the compressor and switching both feed wires, as the compressor is 240 volts.

I have to assume the unit is wired correctly internally, but you may want to check it.

It may be easier just to replace the breaker if it is defective, because you need special equipment and knowledge to test a breaker.

The load may be too great for the breaker also, how much load is running? With two 70 amp breakers, what are these for, and how many other circuits are on the main?

Amprobe could help. If you don't have one, With Breaker and Power off, Disconnect load
Side of Contactor in Outside Compressor Unit, Turn ON Breakers, wait 5 seconds, Set themostat to cool, Breaker Doesn't Trip. Problem may be in outside unit.
You are saying Unit and also saying 2 70 Amp Breakers, I am assuming separate
Units, Condensing Lines? Heat Strip?
Thermostat control due to defect or misswiring, May want to heat and cool at same time.
Tripping breaker.