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master cylinder again?

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jvandyke

jvandyke

Luke Skywalker
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Pedal went soft while driving. Pumped back up, went soft again a bit later. Bled them all. Soft before I reached the end of the driveway, by soft I mean it goes to the floor and you have NO brakes.
I've rebuilt this thing 3 times now. The condition of the bores looks fine to me, I did hone them lightly. I need to be sure I haven't overlooked something obvious, an adjustment?
Otherwise I'll rebuild it again I guess. I'd send it off for sleeving but I'm just not sure that's the problem. Last time I had it off and on the bench it was hard as a rock and stayed that way. Could be eating the main seal on rough bore walls?
It's a 3/4" so my choices are limited.
 
When you rebuilt the master last time did you do a careful job of cleaning the piston grooves? If the grooves are not COMPLETELY clean they can catch the lip of the cup and keep it from sealing against the cylinder wall.

Bill
 
I'm not exactly sure I know what you mean by piston grooves but I've decided there's not much to loose (but my time) in cleaning thoroughly and rebuilding again. Careful inspection of seal shows it is torn. Maybe just another seal then. Too bad a 3/4" seal like this is hard to come by. Wonder why it happened though.
 
The little holes inside the reservoir are easy to overlook and are important. I run a shipping tag wire thru them. If they are plugged they can cause some of your symtoms.
KA
 
Yes, you're right about the little holes, didn't know they were even there at first, here's the MC when I first opened it up last year. I have become quite intimate with this MC. I edited my post above since I found a tear in the seal.
Looks like some odd wear along that edge. Hmmmm Getting pushed crooked? Rubbing on something rough in there?
I think I'll rehone and replace the seal and see where I am then.
tornseal.JPG
 
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