Back brake problem

[fatboyslim]

I was out climbing this hill on my yz125 and crashed. After the crash my back brake had no pressure and didn't work. I've rebuilt the master cylinder and put new seals in the cylinder where the piston is. I have bled and back bled so much that there is absolutley no air in the system so could it be the brake line why it's not holding pressure?

 

[whenfoxforks-ruled]

If its not holding pressure, that means it has a leak. It would be leaking, and visible. If your bleeding method has issues, it will never get the air out. On the other hand, rubber brake lines do fatigue, and it does not take much of a weakened area to make the brake soft. Stainless braided are much better!

 

[TRexRacing]

Does it respond to pumping the pedal and then go away?Or is there nothing at the pedal?Have you tried bleeding the system with the caliper unbolted and higher than the master cylinder?Remove the caliper and let it hang higher overnight so any air trapped can move to the caliper and bleed it there.Be sure to put something in between the pads so the pistons don't pop out of the caliper.I use a wrench handle and some masking tape to hold it.
Did you check the rotor to be sure it wasn't bent?A bent rotor will push the piston back into the caliper requiring pumping the pedal to get any pressure.And it will go away soon as you start moving again so you have to pump again.

 

[fatboyslim]

Ya when I pump it there is no pressure what so ever at the pedal. I hooked the caliper up to my front brake sytem and it pumped the piston out like it should then hooked it back up to the rear system and the same thing as before, no pressure.

 

[TRexRacing]

You've diagnosed where the problem is.Disassemble the master cylinder and see what's wrong.Maybe toasted a seal installing it.Maybe installed a seal backwards.

 

[rmc_olderthandirt]

Does the master cylinder pump brake fluid when you bleed the brakes, or are you using a vacuum bleeder? If the master cylinder does pump fluid then I would suspect a major problem there.

Even if it pumps fluid during bleeding it is possible that the master cylinder was damaged in the crash and needs to be replaced. Visually inspect the cylinder, look for any sign that the cylinder has been deformed in any way. It is possible that an impact has made the cylinder not round anymore and it will never seal well enough to develop pressure.

Rod

 

[fatboyslim]

Well when I put fluid in it and disconnect the hose from it it pumps fluid out the port where the hose connects to fine. Could it be that it just can't pump up enough pressure when the system is closed?

 

[whenfoxforks-ruled]

Sure sounds like a ripped seal.

 

[rmc_olderthandirt]

I had a similar issue once, turned out that I had put the master cylinder back together incorrectly. If I recall correctly, there is a free floating "plunger" that has a seal at each end and I put it in backwards.

In my case, however, the master cylinder still worked when I took it apart (just leaked) and didn't work at all when I put it back together so I knew I had done something wrong.

In your case the master cylinder stopped working after a crash. It is possible that in the crash the brake was applied so hard that it ruptured the seal in the master cylinder, and then you put it together wrong so it still doesn't work.

Do you have a manual that shows how the parts go together? If not, take a look at one of the online parts diagrams, it will show the exploded view of the master cylinder. Study it, then take your master cylinder apart and make sure you put it together properly. I can tell you from experience that the plunger will go in the wrong way and not work!

Rod

 

[fatboyslim]

After the crash all i did was look at the plunger and seals and they weren't ripped or folded or anything and i'm positive i put it in right cuz with the new rebuild kit i put it in before and it worked until the crash. And ya i double checked the manual and it's fine.

 

[julien_d]

After the crash my back brake had no pressure and didn't work. I've rebuilt the master cylinder and put new seals in the cylinder where the piston is.

So you actually did the rebuild before the crash, and it worked until then and not since?

Anyway, if you tested the caliper while removed from the rear loop and it's good, then the problem is the line or the MC. I vote for MC, regardless of if it looked ok to you when you took it apart.

 

[TRexRacing]

Ya when I pump it there is no pressure what so ever at the pedal. I hooked the caliper up to my front brake sytem and it pumped the piston out like it should then hooked it back up to the rear system and the same thing as before, no pressure.

Like I said earlier you've diagnosed where the problem lies.Fix it.