Chasing a brake bleed problem

[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 

[royhall]

Good find Trig. I have noticed on my most recent build that many pattern parts have gone really shit. So much so that some of them aren't worth making as they just end up scrap. Control cables being a case in point.

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[Trigger]

Off to a customer tomorrow to replace the brake switch on a bike that i sold and now the customer is having brake problems. DSS has a lot to answer to.

[AshimotoK0]

Off to a customer tomorrow to replace the brake switch on a bike that i sold and now the customer is having brake problems. DSS has a lot to answer to.

I don't think they care mate. I have offered to analyse stuff in the past to endorse the repro rubber engine/inlet rubber/airbox parts they sell  by getting chemical analysis done but you just hit a brick wall ..like " Materials used are our suppliers / our commercial information". So I voted with my feet on their repro rubber parts.

[paul G]

I bought a set of handle bar grips from them for the CD175 sloper.
Cut the old ones off and came to fit the new ones and the throttle one was incorrect and way too small.
I rang them up and explained that the sloper has a metal tube which goes over the cable control and the grip needs to be much larger to fit this.
They didn't seem interested and said that the part was correct, well when the guys ear had stopped bleeding he took the measurements of my old one and managed to get the correct part as it is used on other models.
GUESS WHAT THE INCORRECT PART IS STILL FOR SALE ON THEIR WEB SITE.
Not interested in changing it, so beware anyone buying the grips for a CD175 sloper THEY DO NOT FIT

[Oddjob]

I'm not 100% sure but the thread on the oil pressure switch looks to be the same as the brake light switch, maybe one of those could be adapted to suit if a genuine switch proves illusive.

[McCabe-Thiele (Ted)]

Aren't they opposing switches one switches on - brake light
, the other switches off - oil light goes out?

[honda-san]

Completely different switches:-
Brake switch, parallel thread, relies on the sealing washer to ensure no fluid leak between it and the brake junction. Oil pressure switch, taper thread, relies purely on interference when tightened (effectively wedged) in to the crankcase top on a CB750 (or oil pump body on a 400/4, etc) to ensure no oil leaks. Different terminal arrangements too (two wires to brake switch, one to oil switch).

[Oddjob]

Yeah good point, forgot about it being a taper thread.

[Trigger]

Different pressure regulated diaphragm between the oil pressure switch and the brake switch