Cooling system pressure test...coolant leaking from overflow tank
 

I rented a cooling system pressure tester from autozone and used it tonight. I noticed that after I get to 12lbs of pressure the coolant starts to leak from the coolant overflow tank. Its not from the top but somewhere else in the tank. After about 30 mins the pressure drops to around 10. Im assuming there might be a crack in the plastic tank but I'm not sure. The dipstick shows the tank is full though. I replaced the radiator cap and the aftermarket ast cap. Im thinking as I pressurize the system the coolant in the ast is being pushed into the overflow causing it to leak out. I originally tested it to check if I had a coolant leak becuse every couple days I get a low coolant light even though I didn't see any visible leaks. Any ideas what might be going on. Now im worried I have a blown seal. Thanks for any help.

Heres a pic
 

How many miles on your engine? Is it an original engine or a rebuild?

After the coolant recall your RX-7 should be equipped with a 0.9 bar (13psi) radiator cap on the AST. Given the vagaries of manufacturing tolerances and the inaccuracies of your Autozone Test gauge releasing pressure at a indicated 12 psi wouldn't be unusual. As for the disappearing coolant. What does the exhaust look and smell like? You can also take it to a shop to test and see if there is combustion byproducts in the coolant.

First, when you do the coolant pressure test it needs to be done on the AST not on the fill cap like in your picture. The radiator cap will have a spring rated for 12-14 PSI or so and when you pump up the system with that tester it pushes the bottom part of the cap up and forces coolant into the overflow.

It is possible the overflow tank is cracked or damaged. Make sure the tank isn't overfull, you want it about half way on the full/low mark. If it's all the way to the top that's no good.

If you're leaking coolant most times you can smell it or see steam or whatever from the leak.

Dale