brakes fried??

lcc

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#1
OK, dropped my '91 KDX 250 off trailer in transit (first time that has happened in about 25 years of riding). This destroyed my front master cylinder. I bought a replacement from a '94 KX 250 and installed it. Same old mushy lever feel, but it worked. Went riding yesterday and the brakes quit waaaayyy back in the woods. Lever moves, no brakes. So today I bled and bled them. I was never able to get air to stop going through the tube (vaccuum bleeder). So is the master cylinder no good or is the caliper bad? I couldn't find anything loose and no fluid leaks either. Advice?
 
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#2
You've got an air leak in the system if you're continuously pulling air and not fluid (assuming the master cylinder is full).
 
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#3
Yah sounds to me some how there is still air trapped in there maybe drain everything and start over???
 

lcc

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#4
Thanks for the replies, guys. I've fully flushed the system to the tune of a whole can of brake fluid already. So apparently there is a leak somewhere in the system? Could it be in the master cylinder or the caliper or in the lines? I know, not an easy question to ask nor an easy problem to diagnose. I'll keep fiddlin' with it and see what I can come up with. Any more suggestions are welcome.
 
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#5
After doing a major service on brakes like replacing brake lines or rebuilding/replacing a master cylinder, I have always found it difficult to get the brakes bled. The trick I use is to crack the bleeder open and put my finger over the hole while I pump the brake lever. I lift my finger just barely to let some fluid out but maintain some pressure in the system. This has always worked for me except for one case, a bike with a shot master cylinder. Since you bought a used replacement, it has problem been sitting dry without fluid for some time and the seals in the master are shot. Master cylinders are inexpensive and simple to rebuild, I would try that.
 
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#6
A caliper generally either 1) works, 2) works and leaks or 3) is stuck. Very simple part that is very easy to diagnose. A master cylinder is a little more complicated. If the piston is free and doesn't leak, the caliper is probably not your problem.
 
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#7
some things ive learned:
1) try bleeding the breaks with the brake caliper above the master cylinder to allow the air to flow up the lines.
2) do what gmc said, keep your finger over the bleeder
3) make sure the reservoir is full all the time
4) make sure that where the brake cable meets the caliper is tight, ive had air get into the system through the copper seal.