FWIW, here's what the FSM says about flush lines.
CONVERTER DRAINBACK CHECK VALVE SERVICE
The converter drainback check valve is located in
the cooler outlet (pressure) line near the radiator
lower tank. The valve prevents fluid drainback when
the vehicle is parked for lengthy periods. The valve
check ball is spring loaded and has an opening pressure
of approximately 2 psi.
The valve is serviced as an assembly; it is not
repairable. Do not clean the valve if restricted, or
contaminated by sludge, or debris. If the valve fails,
or if a transmission malfunction occurs that generates
sludge and/or clutch particles and metal shavings,
the valve must be replaced.
The valve must be removed whenever the cooler
and lines are reverse flushed. The valve can be flow
tested when necessary. The procedure is exactly the
same as for flow testing a cooler.
If the valve is restricted, installed backwards, or in
the wrong line, it will cause an overheating condition
and possible transmission failure.
CAUTION: The drainback valve is a one-way flow
device. It must be properly oriented in terms of flow
direction for the cooler to function properly. The
valve must be installed in the pressure line. Otherwise
flow will be blocked and would cause an overheating
condition and eventual transmission failure.
OIL COOLER FLOW CHECK
After the new or repaired transmission has been
installed and filled, the oil cooler flow should be
checked using the following procedure:
(1) Disconnect the From cooler line at the transmission
and place a collecting container under the
(2) Run the engine at curb idle speed, with the
shift selector in neutral.
(3) If the fluid flow is intermittent or takes more
than 20 seconds to collect one quart, the cooler
should be replaced.
CAUTION: With the fluid set at the proper level,
fluid collection should not exceed (1) quart or internal
damage to the transmission may occur.
(4) If flow is found to be within acceptable limits,
reconnect the cooler line. Then fill transaxle to the
proper level, using the approved type of automatic
FLUSHING COOLERS AND TUBES
When a transmission failure has contaminated the
fluid, the oil cooler(s) must be flushed. The cooler
bypass valve in the transmission must be replaced
also. The torque converter must also be replaced.
This will insure that metal particles or sludged oil
are not later transferred back into the reconditioned
(or replaced) transmission.
There are two different procedures for flushing
coolers and lines. The recommended procedure is to
use Tool 6906 Cooler Flusher. The other procedure is
to use a hand suction gun and mineral spirits
COOLER FLUSH USING SUCTION GUN AND
(1) Disconnect the cooler lines at the transmission.
(2) Using a hand suction gun filled with mineral
spirits, reverse flush the cooler. Force mineral spirits
into the From Cooler line of the cooler and catch
the exiting spirits from the To Cooler line. Observe
for the presence of debris in the exiting fluid. Continue
until fluid exiting is clear and free from debris.
(3) Using compressed air (under 40 psi.) in intermittent
spurts, blow any remaining mineral spirits
from the cooler, again in the reverse direction.
(4) Pump one (1) quart of automatic transmission
fluid through the cooler before reconnecting.
(5) If at any stage of the cleaning process, the
cooler does not freely pass fluid, the cooler must be
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