Bleeding a Concentric Hydraulic Clutch Set-up





By By Ken Tarry


I recently installed a Mazda M5OD transmission out of a 1990 F-150 in my 1967 F-250. After a long and tiring process, I came up with a foolproof and very simple way to bleed the hydraulic clutch system. This will work not only for transplants like mine, but also for trucks originally equipped with this type of system.


STEP ONE: Bench bleed the master cylinder and the connecting line. Place the master cylinder in a vise or hold it down to a firm surface. If you use the vise, be careful not to over tighten the vise as you can damage the internal workings of the cylinder.


With the connecting hose attached to the end of the master cylinder, use a small object to hold the end of the check valve open. This is very crucial. I used an ordinary toothpick from the kitchen. I softened the toothpick slightly by lightly chewing on it, IT WAS CLEAN!!! Depress the check valve with a small screwdriver or something not pointed and insert the toothpick, LEAVE IT WHOLE.


Place the connecting hose into the reservoir of the master cylinder and fill it with brake fluid. Have an assistant hold the connecting line in the reservoir while you pump the master cylinder. Pump in and out SLOWLY so you do not damage the rubber plungers.


After all the air bubbles have stopped, you can remove the toothpick. This is kind of tricky, as you must do it while it is still in the fluid, or you will allow air to return to the line. Once the toothpick is removed, you can pull the line out and install the rubber and cap to the master cylinder.


STEP TWO: In order to make this job easier, there are two ways to fill the slave cylinder located inside the bell housing. Either remove it and fill it before installing the transmission, or by using a vacuum pump. If you decide to fill it before you install the transmission, then you have to completely remove it from the bell housing. Once it is out, remove the bleeder screw completely. Fill the slave cylinder and move it all around to chase as much air as possible, then reinstall the bleeder screw and reattach it to the bell housing.


If you do not remove the slave cylinder, and I didn't, then here is how to make a VERY HARD job VERY EASY. Use a vacuum pump. Most auto parts stores either sell them or loan them. Read the directions on how to PUMP fluid, not vacuum it.


With the bleeder screw open, NOT removed, insert the tip of the vacuum pump hose into the hole in the end of the bleeder screw. Take a blunt object and open the end of the slave cylinder where the "quick disconnect” hose plugs in. There is a metal center piece and a nylon part that surrounds it. You have to depress the nylon part. Have some spare rags ready, as this can get a little messy. Begin pumping fluid into the bleeder screw assembly. When the fluid begins coming out of the hole where the "quick disconnect” inserts, remove whatever is holding the nylon part open and close off the bleeder screw. YOU MUST REMOVE CLOSE THE NYLON PART FIRST. With a vacuum pump, the fluid will continue to move until there is nowhere for it to go, so you will have time to close off the bleeder. However, if you close off the bleeder screw first, you will allow air to return to the slave cylinder, and that is NOT GOOD!!!


STEP THREE: Install the "quick disconnect” line. This thing was named very wrong. It can be very frustrating to install, but here is a little tip. There is a little metal ring inside that holds the fitting into the slave cylinder. If you carefully remove it, you can bend the tabs that lock the fitting into the slave cylinder down a little bit. Be careful as to not go too far, because they could break off. Reinstall the metal ring, make sure the tabs are facing outward, and slide the "quick disconnect” fitting into the hole. Use a small flat screwdriver to push the Teflon lock ring into place; this too can be kind of tricky.


STEP FOUR: Now that is all reassembled, there is probably a small amount of air trapped due to the installation of the "quick disconnect” fitting. This is very easy to bleed out now.


Have an assistant sit in the truck. Have the person depress the clutch, but DO NOT LET THEM PUSH IT TO THE FLOOR. A hammer handle under the clutch pedal keeps the pedal from going all the way down. You are the one opening and closing the bleeder screw, and you will give the commands. On a side note, bleeding a clutch is NOT like bleeding brakes…DO NOT PUMP THE PEDAL. Repeat… DO NOT PUMP THE PEDAL.


Tell your assistant to push the pedal down, when the pedal is down, open the bleeder screw, and allow the fluid and whatever air is trapped in the system to run out. You can use a clear hose and a jar to catch the expelled fluid, but if you don't, keep some rags handy. Close off the bleeder screw and have your assistant allow the pedal to come back up.


Do not let your assistant depress or return the clutch pedal until you tell them to or you will allow air back in the system Repeat the bleeding until all the air is out and you have clear, air-free fluid flowing from the bleeder screw. Make sure you keep the reservoir filled as well, if you let it get empty, then you will have to start all over.


If you follow these steps, then what is often referred to as a HARD job, will be very easy to do. I had mine bled in about a half-hour.

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