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Chasing a brake bleed problem

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Trigger:
After building a bike with new brake lines, banjo bolts, washers, piston seal, brake light switch, stainless steel piston, brake nipple, Tourmax master cylinder kit and using DOT5 silicone brake fluid. The brake would pump up fine but, leave it for 5 minutes and the brake lever would come back to the handle bar.

I have never ever had a problem with bleeding brake in my life, i know that it is a system and must be treated as such. Even had a mate turn up with his all dancing suck and blow bleed kit and no success. I changed the master cylinder kit and started the bleed procedure again but, still the same results. So, changed the master cylinder, another set of brake lines, same results. Changed the brake caliper with another new seal and piston trying to work out what was going wrong but, the same results. Time to put it in the back of the workshop as it was driving me mad.

Was out for a spot of breakfast with Chris Rushton and he was telling me about one of his 400's and he had changed his brake light switch for a DSS replica one and the brakes had been shite since.

Got the bike back out on the bike lift yesterday and removed the repro switch and fitted a old Honda one. A couple of bleeds and the brake is perfect now.

McCabe-Thiele (Ted):
Wow so it ends up as the switch causing the problem very informative read.

Brakes sometimes do seem challenge logic. Back in the 1960's I helped a mate relocate his brake servo as it was a clutter under the bonnet. Brakes were fine before we started. Made new brackets,  moved servo - could not get a decent pedal. We concluded wrongly it was due to the change of height relative to the master cylinder.
Put the servo back where it was - no change.
We had used the best part of half a gallon of brake fluid so gave up for the day.

Following day we started by clamping up brake hoses at the wheel end one at a time. Fourth hose lucky a rear wheel cylinder was letting in air but not leaking fluid. We never did move the servo.

Matt_Harrington:
Amazed the switch could cause this issue and hence it took you ages to resolve. Good ol' pattern parts, eh! Thanks for posting.

davidcumbria:
Good to know this. What a tortuous path to finding  the solution. Would have driven me mad as well but great that the solution was something definitive.

Trigger:
Always drives you mad when you assume a new part is OK  ::)

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