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If you have a bit of sponge left it might be air from the proportioning valve. I've had shops bleed my brakes and ignore that valve. It's left me with a little sponge.

good call.
I was just hopping back on to report that the pedal went kinda flat and spongy again. I ended up breaking out the vacuum bleeder kit I picked up from Harbor Freight several years ago and pulled fluid through all the lines until it was all fresh, including the LSPV.

wherever it was, there definitely seemed to be air in the system somewhere. it holding pretty solid now.

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How often are you topping off the fluid? If you are at all, it has a leak somewhere, brake fluid doesn't evaporate <img src="/forums/images/graemlins/kewl.gif" alt="" />

Try checking where the pushrod connects to the pedal inside the cab. If it is damp, there is at least one of the leaks. I've rebuilt many master/slave cylinders over the years, only costs a couple bux <img src="/forums/images/graemlins/cheers.gif" alt="" />

I wasn't topping off any fluid at all, except when vacuum bleeding the system, and that was just to make sure I wasn't draining the reservoir.

I have had a leak before where you mention, in fact, that's how I came to replace the master cylinder a little over a year ago.

so you've had good luck with rebuilds? I've never tried one before. guess you have to have the small cylinder hone? isn't that pretty much all there is to it?

msg - '87 xtracab