Spongy rear brake when hot

[Paul_]

I have searched the threads and cannot find any answer to this.

When cold the rear brake is fine, but during a ride it gets progressively spongier. It still works but requires a much harder prod to get it to work as well as when I start my ride. I have thought about replacing the rear brake fluid and bleeding this through to see if it will improve it but from searching the rear is always a weak spot of the bike as discussed here and on other forums.

I do sometimes drag it in town when riding in very slow moving traffic/very tight bends but don't lean on it very hard. Fluid is original as far as I know and not due for replacement on the service schedule until next year, although this will be replaced sooner as well as the front brake and clutch fluid.

I am not rider that relies on the brakes and prefer to run at 'the pace', being as smooth and fluid as possible using the brakes as little as possible unless really needed if I mess up! The brake is not binding or anything, I have checked.

My thought is that the rear reseviour is right beside the exhausts and will soak heat from them pretty well and believe this could be most of the problem. Is there anyone on here that has found a solution to this or has experienced similar? i.e is there anyway of covering this reseviour and what with?

[Dukedesmo]

Bleed out the air and if it's not been done recently change the fluid.

My 916 suffers from the same problem and the pedal travel also gets progressively longer until it doesn't work at all, there are no leaks it just seems to get air in it rather quickly.

Doesn't help that the master cylinder is so close to the exhaust and, in fact the rear mounting bracket bolt also holds the exhaust mid-pipe.

That said I rarely use it anyway 'coz it's crap...

[Starter Sprag]

Both hydraulic oil, and air, expand with heat.

But, the oil will not compress, the air will, you have air in your system !

Drain the old oil out, and put new stuff in, old oil will absorb moisture, reducing performance and causing corrosion

It does take time though, to completely 'bleed' all the air out of the system

[Nickj]

Not even sure if mine still works!!

[utopia]

If its spongy when hot but ok when cold I think that moisture in the fluid (rather than air) is likely to be the problem.
Brake fluid is hygroscopic (ie it readily absorbs atmospheric moisture). While the fluid remains cold, the moisture is present as liquid, and is incompressible so braking performance is not affected. When the fluid heats up, any moisture within it turns to a compressible vapour, causing sponginess.
Change the fluid. Do it on a warm, dry day. Use only fluid from a newly opened container, and you might as well use whats left to change the fluid in the front brakes and clutch at the same time. Its not worth having a half full container on the shelf for next time, cos it will have absorbed atmospheric moisture by the time you come to use it again. If you must save some, decant it into a smaller, clean dry glass bottle, filled to the brim and tightly sealed. I find empty half bottles of whisky are ideal....the thin ones fit very neatly on the shed shelf too......:look:

[Paul_]

Fluid change on the cards then, well explained as always. Thanks all!

[Darkness]

I find empty half bottles of whisky are ideal....

Good advice about old brake fluid with water in it and why it doesn't work, but I find full whiskey bottles are more useful!!!!!

:spin::spin:

[Dukedesmo]

I find empty half bottles of whisky are ideal....the thin ones fit very neatly on the shed shelf too......:look:

Just be careful which bottles you take a swig from whilst 'relaxing' in the shed... ;)