Renewing the trans fluid is not as simple as changing the engine oil. Too little fluid will churn and induce air. Too much will force it out the breather. For model 124 (probably similar to 126) you are supposed to pour in some amount less than full, run the engine and shift through all gears to move fluid into the servos, then add the rest. I don't change my fluid/filter myself. An automatic transmission is too much for me to screw up.