Engine Toro with Problem with Kohler engine

My Toro 518XI mower has a Kohler engine with a problem, engine Info--- Family SKH624U1G2RA,,,MODEL CH18S,,,SPEC #62579,,,DISP. 624CC,,, SER.#273241450T. It is loosing a lot of oil. I fixed a leak last year at the cover over the breather reed. Toward the end of the mowing season the top of the engine was covered with oil and the air filter is oily. I took the dip stick out while it was running and there was air pressure. Then I took the oil fill cap off and there was a lot of air being pump out of it. The engine doesn't smoke so I don't think the rings are bad and the breather reed was OK when I fixed the oil leak last year. It starts very easy and runs good. I've heard the Kohler CH18S is prone to blowing head gaskets. Would that cause the listed symptoms and if not what do you all think the problem is?
Thanks
Dana

the 25hp engine was the one with the head gasket issue up until they replaced it with a fire ring type gasket. You problem sounds like a blown head gasket issue. but could also have a leak on top of the engine were the oil is, and if it is leaking very bad can cause lose of vacuum in the crankcase.

ILENGINE said:

the 25hp engine was the one with the head gasket issue up until they replaced it with a fire ring type gasket. You problem sounds like a blown head gasket issue. but could also have a leak on top of the engine were the oil is, and if it is leaking very bad can cause lose of vacuum in the crankcase.

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I can't see any place where oil is leaking. I think it's running down between the cylinders and coming out at the bottom/base of the engine. When I take the oil fill cap off there is pressure but it seems like it wants to jump up and down when very close to the valve cover then when I pull it way from the valve cover there is a steady flow of air coming out. When the fill cap wants to jump up and down is that the pulses of the pistons going up and down making vacuum and pressure? If so there seems to be a lot more pressure than vacuum. Does the fuel pump work off vacuum or pressure or both?

Twin cylinder engines don't produce much vacuum, so it may be somewhat normal since by removing the dipstick you are allowing air to enter the crankcase, causing more pulsing. The Fuel pump could be vacuum or mechanical. If the fuel pump is located on the valve cover than it is mechanical. If it has three hoses attached to the fuel pump then it is vacuum.

I checked the oil fill and the dipstick separately. The fuel pump is mounted on the valve cover and has 3 hoses so it would be vacuum operated. I thought they worked off vacuum but since the engine is making so much more pressure than vacuum I wasn't sure and thought they might work either way. Thanks for the info.