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Posted by D Sherman [72.47.9.228] on Thursday, June 09, 2011 at 11:21:21 :
I'd appreciate some new idea on what could be wrong with the brakes on my m37. Last year, coming down a long hill, I had a weird problem where the pedal pulsated amazingly, like maybe 2" up and down. Sometimes it would do it and sometimes it wouldn't. This year, when I fired it up and started driving around town, the front brakes would lock up within a few blocks and it would take all the engine's power to keep it moving in low gear. The rear brakes were fine so I knew it wasn't that little bleed hole in the master cylinder that can get clogged up.
I replaced all three front hoses, since they were very badly deteriorated on the outside, in case the lining was coming apart on the inside, and that seems to have fixed the lock-up problem. The problem then was that I had a hard time getting any pressure on the pedal. I could pump and pump and sometimes it would firm up, but sometimes not. Usually there was enough pressure that if I put the pedal all the way to the floor, the truck would stop. If I could get it to "grab" at some point, it would stay firm at that level and not bleed down. It sort of acted like air in the system or a bad MC, but I bled and bled and bled it, from all 4 wheel cylinders and I'm positive I have all the air out. I had rebuilt this MC when I got the truck 10 years ago, but the cylinder was pitted and I knew it leaked a little bit.
Not being able to think of anything else to do, I put in a brand new MC and expected it to solve my problem, but it does exactly the same thing. Very soft pedal, just barely enough to stop the truck, but once in a while with much rapid pumping I can get it to firm up, at which point it stays firm as long as I keep the pedal part way down. When I bleed it, it pumps fluid out smoothly at all wheels.
I went around and adjusted all the brakes, thinking maybe I'd backed some of the shoes way too far off when I was having the lock-up problem, but all seemed pretty close to right. I do have one front shoe where I can't quite get it to not drag at all, and if I turn the lower adjuster even a very slight distance in either direction, it locks. Also there's one rear shoe where I can't turn the lower adjuster at all, but the brake isn't dragging so I just left it. All the drums are warped to some extent, but I don't care about that.
Last night I took it up and down the steepest road around here -- one that's so steep I can't get up it in 2WD even when it's bare and dry -- just to see if I thought I could make do with the brakes the way they are. With much pumping I did have enough braking action to slow the truck, and no one wheel skidded so I think they are pretty evenly adjusted, but it wasn't something I'd want to do regularly.
What else could be wrong? It sure seems like it has to be either the MC or air in the system, but with a brand new MC acting just like my old rebuilt one, and with much bleeding and not the slightest bit of air coming out, it seems like it rules those things out. There's no leak that I can see anywhere, and if there was, the MC should go empty pretty fast, which it doesn't. The only thing left to do is pull all the drums and see if there's anything funky going on in there. The trouble is I can't think of anything that could happen inside the drum to cause a very soft pedal like this.
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