Hard Pedal, Dragging Brakes

[M543A2]

I had a problem with my M51A2 with the brake pedal being hard, right at the top after a few brake applications. The brakes were dragging, building heat in the drums. I had replaced a stuck master cylinder with one that seemed good off a parts truck. I had disassembled it and cleaned it before putting it on the truck. According to manuals, the most likely cause of the problem is that the master cylinder piston is not returning far enough to let pressure bleed off. My piston was returning fully, so I had to look for other causes. What I found to be the problem was that I had missed that the small pressure bleed hole in the master was plugged. The picture shows the inside of the cylinder. In the large hole in the bottom you can see part of the aluminum cylinder piston. If you can see it as in the picture, the master is returning fully as it should. You can also see a smaller hole behind the larger hole. This is the bleed hole that was plugged. I used a drill type torch tip cleaner with an .030 diameter drill bit to carefully clean out the small hole. You can run the bit down through it with the cylinder on the truck with no problem. The brake pressure bleeds off fine now with no more hard pedal and dragging brakes. It was a really pesky problem until I figured out the cause! Hopefully my experience will help someone else.
Regards Marti

 

[gimpyrobb]

Marti, many guys are having this problem, just not posting about it. I'm sure you have already helped 5 people.

 

[Defendr200]

I had the same problem on a 1983 nighthawk 550. It had a hydraulic clutch and would start slipping the more times you pulled the clutch in. This little hole is extremely important. It took me close to 3 weeks asking people and looking on forums with absolutely no answer. I just got lucky and saw that little speck of dirt blocking that hole cleaned it out and it worked perfectly.

 

[Recovry4x4]

That is the compensating port and as much as I point it out, it will be only half as useful as you post with an actual pic. 1000 thanks Marti.

 

[gimpyrobb]

Bump for Mark

 

[Somemedic]

Bump for Mike Walker...

 

[vtdeucedriver]

That is not always the case...............I had a brake booster go bad that did the same thing on my M52A2. Had a hard pedal but only the right rear axles got hot..............go figure

 

[vtdeucedriver]

that being said, my M151 is having the same issue.