Posted by Gordon Maney on Sunday, July 27, 2003 at 8:15PM :
In Reply to: M-37 - Problem bleeding brakes! posted by Dan Shockley on Sunday, July 27, 2003 at 11:30AM :
The essential elements of bench bleeding can also be achieved with the master cylinder mounted on the truck. Simply depress the pedal slowly with the fill plug or cover removed. Bubbles will rise to the surface of the reservoir. Keep doing that until all the bubbles stop. Add fluid as necessary.
The textbook scenario for bench bleeding involves a line or lines running from the discharge port(s) on the master cylinder, directing the discharge flow back into the reservoir. No waste, no mess.
Most people don't have such lines and don't care to make them up. You can also pump the fluid into the air, which is a messy proposition.
Once you have pumped the pedal slowly with the master cylinder cover removed, until no more air comes to the surface, you should be ready to begin bleeding.
Another thing that can help bleed a system that is filled with air is to use your adjusters to move all brake shoes fully out and tight against the drum, locking the drums in place. Years ago I used that method on long trucks and school buses. School buses were really bad, they were so long. There can be a lot of air in a line that is as long as the bus.
That way, no master cylinder piston motion is wasted on shoe movement before system pressure rises. That enables you to get maximum velocity during the opening of the bleeder valve.
Some books and manuals, and some shop lore, will say you should begin with a certain line; the long one, the short one, etc. My experience tells me that it really does not make any difference. The two things that really count are the bleeding of the master cylinder first, and then, if necessary, locking the drums with the adjusters. On the 1-ton Power Wagon, WWII 3/4 ton, and M37 (I have not worked on a WC 1/2....) remember to use both adjuster cams and lock both shoes out for best results. I am not going to get a book out just now, but I would guess the 1/2 ton has the same kind of arrangement, or similar, at least, with a fixed anchor brake and two adjusters.
Put a hose on your bleeder and direct your flow into a glass jar. The very last stream you may wish to direct into the air, so you can watch it....
Have fun....