By AndyW 11/2004

A check worth carrying out regularly, after all your brakes are pretty crucial!:

Several months ago I was down at Pete59's...we were in his back garden with Pete giving me a demo of his straight through exhaust pipes! .

Anyway, in conversation he mentioned that he was worried about the front brake behaving oddly at times.....pull lever and buggar all happened, little or no resistance and lever pulling back to the bars!......and very little braking force. Release and retry, invariably ok. (think that was the description from memory).....

Chatting through the possibilities when I suddenly noticed that the brake fluid reservoir appeared to be full to the brim, right up to the lid, way passed the high level mark......further investigation showed that the rubber diaphragm within (1 in the diagram below) was 'distended' down in to the reservoir. Removed and the 'pleat' tweaked back in to shape and all was well.

Never thought anything more of it.........put it down to Pete probably having over heated or boiled the brake fluid on a drag run with the diaphragm drawn down excessively when the brake fluid cooled.

So to the point.........tinkering with my bike today and found the exact same thing. Not had the braking problems so presumably only just happened on the bikes' last outing.

The diaphragm is bound to get drawn down as the brake pads wear as there will be more fluid in the callipers with the pistons further extended resulting in there being less fluid in the reservoir itself. This is the purpose of the diaphragm i.e to keep air out of the actual system and keep the reservoir mostly fluid filled - brake fluid is (absorbs water/moisture) so the brake fluid needs to be isolated from the atmosphere. However if the diaphragm becomes distended/misshaped maybe that stops the master cylinder functioning properly?

Possibly just a coincidence with Pete's braking problems...but worth checking regularly ......loss of your front brakes could be nasty!!!

See what I mean - fluid level way up in the cap somewhere

'Distended' diaphragm -NB: the strange grey/white appearance of the rubber diaphragm
is due to flash light reflecting from the wet surface

....and back in shape

...and fluid level back to normal (bike on side stand and bars turned)
 

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