A common problem with EVO's is base gasket leaks. Many times just replacing the base gasket doesn't solve the problem. The base of the cylinder can change shape over time. Heat, cylinder stud tension and cylinder compression can all take its toll. Lots of base gasket leaks can be attributed to this. The cylinder studs firmly hold the cylinders down in the 4 corners. When the cylinder heats up it expands and the centers between the cylinder studs distort. When changing the base gaskets on an EVO it is good to put the cylinders on a lapping plate and check the bottoms for flatness. Many times you will see that only the stud holes make contact with the lapping plate.
This is a lapping plate for EVO cylinders. You can see lapping compound on it.
For this example I used a blue sharpie to cover the cylinder base. The cylinder base looked flat to me.
I set the cylinder on the lapping plate. While pressing down, I turn the cylinder and the lapping compound "sands" the bottom of the cylinder base.
After wiping off the excess compound, you can see the high and low spots. The blue sharpie is left in all the low spots where is doesn't hit the lapping plate.
You can see the low spots are between the cylinders. At these low spots there is less pressure against the gasket causing a poor seal. Note the low spot here is all the way from the edge to the cylinder sleeve.
I covered the base with blue sharpie again and repeated the process. You can see I removed more of the high spots but it is still not flat.
I repeated the process until all the low spots were gone. The cylinder base is now flat again. This will allow it to apply equal pressure on the gasket and reduce its chances of leaking.