Rear brake problem

[moloprintz]

We're on our way home Saturday after a low speed curvy ride.
I tend to use my rear brake for control in the curves.
If I have the proper RPM and I've still got too much energy going in I'll use the rear brake.
Late in the afternoon I applied rear brake and the pedal went all the way down with no effect.
No problem, light front brake with engine braking and everything’s nice and smooth.
I tried the brake pedal a couple of times with the same result each time.
After a refuel and rest stop the rear brake works fine.
Sounds like a master cylinder problem.
The question is do I buy new, low mileage used or a rebuild kit?
If a rebuild kit, do I need to hone the cylinder wall?
Has anybody else had this problem?

Mike

[cnc]

When was the last time you changed your brake fluid? Lots of moisture in the fluid can cause serious brake fad when the fluid gets over heated and the moisture boils.

[moloprintz]

Good point.
I did the front and clutch in the spring.
But it's been more than a year for the rear.

[ringadingh]

You really don't hear of any master cylinder problems, Id start by changing the fluid. If that doesn't fix it the replace the master cylinder. I see them on Ebay quite reasonably priced.

[themoonpie]

I just returned from the Redwoods rally. When we were Riding over the Siskiyou's, Very High altitude and 80+ temps. My rear brakes did the same thing. All the way to the bottom with no sign of wanting to pump back up. Oh riding on chip sealed road also. Thought it might have been a tar coated rock I picked up inbetween the pads. But it sank to the bottom. likewise with light front brake and compression I was able to get to the bottom of the mountain slowly on fresh gravel trailing a loaded logging truck.. Nail biter with my co pilot on board. But when we reached range land pedal came back to normal and everything was fine all the way back to Washington State. Fluid was changed a month before this rally and had several rides before we left. I think Heat and altitude may have been my Factor.. Any other thoughts

[AlabamaNomadRider]

Heat and altitude could definitely be a cause of the failure. Possible air in line also.

[cnc]

In my mind there are only 2 reasons altitude would have an effect on hydraulic brakes, OK 3 if you count the fact that what goes up must come down and you are using your brakes more and heating everything up.
1. Any moisture in brake fluid will boil at a lower temp as altitude increases.
2. Any air bubbles in the system will expand as altitude increases and the enlarged air bubbles will make the fluid highly compressible.

I would suspect that either of the above could happen even on newly bleed brakes. Being hygroscopic, if a previously opened can of brake fluid was used one could have as much moisture in the system as before they were bleed. And /or when the brakes were bleed, minute air bubbles were in-trained in the fluid. Not enough to be noticed much at lower altitude and reasonable braking was achieved but climb high enough and they show up.

[ringadingh]

I doubt that altitude would have a serious effect on the brake system, if it does they would be in pretty poor shape regardless of the altitude. Its probably a bad seal in the master cylinder or the caliper.

[themoonpie]

Funny thing about thw whole incident is that they workrd perfectly all the way to our lunch stop and all the way to the top. not a lot of braking going up but it was hot and up there in altitude 4000 ft . it wasn't until we got on that chip sealed road at the top . then they just went to the bottom there was no pumping that would help. I never tried them again until I hit range land at the bottom. very slow decent with engine compression and front brake. When we came to the end of our mountain road stop sign. They were full up again and worked pefect the rest of the trip back to Seattle. not sure

[classic1]

The same thing happened to my 06. It has happened two times on the same hilly, winding road, going in the same direction a couple of years apart. Each time after stopping on level ground, the pressure would build back up with no after effects. The first time, I changed the fluid. The second time, I didn't do anything. The last time has been about two years now.

[Idaho]

Uh, altitude is not the problem here. Many of us out West live at altitudes above 4000 feet, myself included. Saturday rides in the mts can go over passes in excess of 7000 feet with no problem. Change that fluid.

[moloprintz]

Fluid changed.
We'll see what happens.

[ringadingh]

Ive ridden a few times at over 7500 ft and never experienced any brake problems at all. Hopefully with the new fluid your all set to go.

[macmac]

If anything the rear caliper is sticking on, not letting go . That will boil BF and cause it to gas out. Upon cooling the gasses condence and you get a full pedal back.

Any system with a 'little air' will cause the pedal to creep down cold. I doubt there is a master problem at all.

Calipers are fairly easy to rebuild.