Topic: Sudden loss of brake pressure

[evanphi]

I noticed this yesterday, and it happened awhile ago too, but only once before.

I'm riding along and after 20 minutes or so of riding, I need to apply some front brake. Two fingers reach out, pull the lever, and it goes straight back to the bar! No hydraulic pressure at all. Then two quick pumps of the lever and it progressively feels normal again, and no further losses of pressure.

No leaks in the system, plenty of fluid (put some more in after the last time it happened).

[Pat_at_APE]

Time to rebuild or replace the master cylinder?   

Pat at APE

[DaveBarbier]

Do you have the spring installed on your brake caliper pivot arm? Is it adjusted properly? Without that spring the caliper can vibrate left to right and push the piston in the caliper. A few pumps and it's back where it was.

[evanphi]

Time to rebuild or replace the master cylinder?   

Pat at APE

did it 2 years ago.

Do you have the spring installed on your brake caliper pivot arm? Is it adjusted properly? Without that spring the caliper can vibrate left to right and push the piston in the caliper. A few pumps and it's back where it was.

yep. installed properly.


like i said this has happened only twice, the last time yesterday and the first time about 3 months ago.

[flybox1]

Time to rebuild or replace the master cylinder?   

Pat at APE

did it 2 years ago.

+1
What was the condition of the MC bore?  Any pitting/debris in there will quickly wear the soft parts.

[evanphi]

Time to rebuild or replace the master cylinder?   

Pat at APE

did it 2 years ago.

+1
What was the condition of the MC bore?  Any pitting/debris in there will quickly wear the soft parts.

Perfectly smooth. I lightly polished the bore myself.

Also, as I said this is a very rare occurrence.

[flybox1]

That's scary, Evan 
I remember having to pump the brakes first on my grandfathers Barracuda before beginning to stop 

[754]

I had this happen once, as I pulled into a place to stop the front brake lost pressure and went to the handlebar, I completed the stop with rear brake.
 When I resumed riding, it seemed to work normally and has for over a year since.  Strange !

In your case, I would change fluid or open the MC to rinse out any bits that may be causing leakage.

[evanphi]

It sounds like a brake hose issue. Perhaps the hose is old enough to decay from time and thus losing its ability to withstand the MC.

I would think it would occur more regularly, but I would still investigate the condition very closely. At a minimum, I would purge and refill the system with 100% new fluid and capture it for examination. Brake fluid is cheap and 45 minutes is cheap insurance where your safety is concerned.

Not hose. Brand new one-piece stainless from Slingshot.

[DaveBarbier]

I still think the pad is working itself back into the caliper. Either through vibration or the caliper moving back and forth. Can you post some pictures of the caliper/disk setup?

[evanphi]

[riffman12]

perhaps you just caught a poorly timed air bubble in the line? Try bleeding?

[DaveBarbier]

Yeah weird, I'm not sure. If it were me I'd probably disassemble the system and give it a good look over. Randomly having no brakes would terrify me, no matter how infrequent.

I'd assume you'd mention if you have any rideability issues like vibrations or wobbling in the front end due to a warped rotor or misaligned wheel or something.

[evanphi]

Nope. No issues with rideability otherwise. Brand new front end (fork tubes and roller head bearings). No vibrations.