The basic rule for flushing a transmission is this: If you haven't done routine service of the trans fluid over the time that the vehicle has been in your possessin, DON"T DO IT. Because the fluid is a high-detergent, low-viscosity oil, it WILL scrub out whatever is still holding the transmission together, when you remove all the old stuff, and replace it with new. If you have serviced it with simple drain-and-fill or filter-drop techniques, you should continue to do so. If you have not had it serviced, it's due for its first service, and you use a quality, exchange-style system, it will do fine, and have no issues after service. Continue to service the tranmission in this manner, and enjoy long service life. Because of the design of the fluid circuits in an automatic transmission, the exchange of fluid is actually very close to 1:1, and the only intermixing occurs as the fluid is forced through the torque convertor. The process only works if the cooler circuit is where you tap in, and only if the pump is allowed to run the fluid through the cooling circuit. If you flush a 727 while it runs in "Park," you will not be able to flush the convertor, the pump will not circulate the fluid through the cooler circuit, and problems will immediately result. There is nothing inherently wrong with the process, but it does require an understanding of the process and the equipment. It also requires knowing when NOT to recommend the procedure.
