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I have a 75 f350 tow truck when i back up or do a u turn and start going foward again I loose all my brake pedal for obout 4 pumps.I have replaced the master cylinder 3 times everything in the back is brand new,I have only one new caliper in front,and I have tried to bleed over and over and over I have run out of ideas can anybody help.
I sounds a little like something is not properly secured on the rear shoes assembly. I would get a diagram and go take a really close look at the springs and retainers and the order that they go on.
Are you shure the primary and secondary shoes are in the right position?
Another suspect could be the 'landing pads" on the rear brake backing plates. There are three or four flats that the shoe 'lands' on. I have noticed these tend to wear a bad groove after 10 to 20 years and will interfere with the brake operating properly. Your pads may be hanging up in these grooves, after being applied in the reverse direction and are taking a few pumps to re-seat. Anyway, you can either replace the backing plates or weld up the pads and grind them back down to a smooth surface (somewhat tricky to get all at the same height). A very small dab of grease is placed on this metal to metal contact area.
Now that you mention it. I've actually welded up some of those grooves you are talking about, and then filed them flat. At the time I couldn't find new backing plates.