mactiredearg
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Awhile back I had the non-releasing brake issue that some of the A3 owners are running into (http://www.steelsoldiers.com/showthread.php?88172-M109A4-brakes-locked-up!). I finally got it figured out thanks to a lot of suggestions on here (particularly peashooter) and wanted to post some pictures of the issue so other A3ers will know what to look for. I also thought I'd cover part of the master cylinder replacement with some pictures.
My issue was that the brakes were not fully releasing after each press of the brake pedal. At it's worse I could go from normal press to completely solid pedal in about 10-12 steps on the brake pedal. The only way to get the brakes to release was to roll under the truck with a wrench and unscrew the bleeder valve on one of the airpacks enough to drain some fluid.
The cause of this issue was the brake pushrod falling out of the cup in the face of the master cylinder. It then rested on the face of the master cylinder and would not let the master cylinder piston return far enough to expose the return port. So in effect I was pumping the brakes without releasing any pressure until it reached max pressure and the pedal wouldnt move.
Here are:
a picture of the pushrod
the master cylinder piston face (note the marks from where the rod was hitting it while out of place)
a picture of the rod in the truck when it has fallen out of place (see how the adjusting nut is touching the bottom of the boot)
a picture when the rod is in the correct position (see how the adjusting nut is touching the top of the boot and is more level).
The short term (get yourself home) fix is to roll under the truck with a wrench and loosen the airpack bleeder valve and let it dribble until it stops then get towed home. This release will get you maybe 8 or 10 stomps on the brake pedal before you have to do it again.
The long term fix (once home or if you have enough time in the field) is to remove the pushrod and adjust it to make it longer so it will quit falling out of the cup. You will need to test the adjustment to make sure it keeps the pushrod in the cup and doesnt over compensate and cause the same issue by pushing too hard on the piston.
Here are the only pictures I have of removing and reinstalling the pushrod. It's about the easiest piece of work you can do on the pushrod. Just pull the cotter pin, slip the spring off the pin holding the pushrod in place and pull the pushrod out.
Note - DO NOT LEAVE THE TRUCK SITTING IN THIS CONDITION. I was _stoopid_ and did exactly that out of frustration. What would have been a simple 5 minute fix turned into a couple hundred dollar repair. The secondary piston in the master cylinder will get STUCK in the fully compressed position if left for too long and you will need to replace the entire master cylinder to get this fixed. Learn from my stupidity.
My issue was that the brakes were not fully releasing after each press of the brake pedal. At it's worse I could go from normal press to completely solid pedal in about 10-12 steps on the brake pedal. The only way to get the brakes to release was to roll under the truck with a wrench and unscrew the bleeder valve on one of the airpacks enough to drain some fluid.
The cause of this issue was the brake pushrod falling out of the cup in the face of the master cylinder. It then rested on the face of the master cylinder and would not let the master cylinder piston return far enough to expose the return port. So in effect I was pumping the brakes without releasing any pressure until it reached max pressure and the pedal wouldnt move.
Here are:
a picture of the pushrodthe master cylinder piston face (note the marks from where the rod was hitting it while out of place) a picture of the rod in the truck when it has fallen out of place (see how the adjusting nut is touching the bottom of the boot) a picture when the rod is in the correct position (see how the adjusting nut is touching the top of the boot and is more level). The short term (get yourself home) fix is to roll under the truck with a wrench and loosen the airpack bleeder valve and let it dribble until it stops then get towed home. This release will get you maybe 8 or 10 stomps on the brake pedal before you have to do it again.
The long term fix (once home or if you have enough time in the field) is to remove the pushrod and adjust it to make it longer so it will quit falling out of the cup. You will need to test the adjustment to make sure it keeps the pushrod in the cup and doesnt over compensate and cause the same issue by pushing too hard on the piston.
Here are the only pictures I have of removing and reinstalling the pushrod. It's about the easiest piece of work you can do on the pushrod. Just pull the cotter pin, slip the spring off the pin holding the pushrod in place and pull the pushrod out.
Note - DO NOT LEAVE THE TRUCK SITTING IN THIS CONDITION. I was _stoopid_ and did exactly that out of frustration. What would have been a simple 5 minute fix turned into a couple hundred dollar repair. The secondary piston in the master cylinder will get STUCK in the fully compressed position if left for too long and you will need to replace the entire master cylinder to get this fixed. Learn from my stupidity.
