Front master cylinder problems

[Tugboat]

Hi we are having issues with the front brake master cylinder on a 1978 CB750 police bike with the dual discs.
> The old and new one do not move enought fluid as you start to brake and the lever will hit the throttle under emergency braking
> Is this common?
> How many cc of fluid does this master cylinder displace (5/8 piston) for the new one? .
> Cannot ride the bike like this please advise
> Thanks
> Lorne

 

[Nunya Bidnes]

Does it PUMP up? Does it get stiffer with more squeezes of the lever?

 

[hinksy]

The twin disc should have the larger 17mm piston , if it is a 14 that is for a single disc . You could need to bleed them , but if you have it may be time to fit a new seal kit

 

[Tugboat]

The new master cylinder is supposed to be a 5/8 -- 17 mm model
It pumps up but the first amount of pressure starts at 50% of the lever travel leaving little travel for applying greater pressure.
Compared to my other 18 motorcycle front brake cylinders this is defective.

There are what appears to be new pads that could be too thin and have to move too far in the caliper to contact the disc.
There are no leakes from the calipers

It could be the old rubber hoses are soft and absorbing the pressure as they expand slightly taking up some of the fluid volume.
New stainless lines have been ordered.

Any other thoughts?

 

[Nunya Bidnes]

I hesitate to insult anybody, but you might review your bleeding routine?

 

[Nunya Bidnes]

Just so you don't think I was being sarcastic, I too, had to practice my "bleeding routine".

 

[Tugboat]

Hi
We bleed race car brakes,aircraft brakes and even ones on antique cars in our fleet.
I am not insulted ,it was a good suggestion for the the semi experienced bike owner.
Pressure ,vacuum,gravity or even the use of the master brake lever have been tried with no change .

Moving the correct amount of fluid is the key to the movement of the lever. We are measuring the CC the cylinder produces. which I think could be ok, if not for the spongy brake lines .
The new ones should be here next week ,iI will keep you posted .

 

[Nunya Bidnes]

Thank you. Right. You mentioned GRAVITY. Only a Theory.

 

[Nunya Bidnes]

U might be on it. I once had a brake line that would only fail while under pressure, by creating a rubber bubble in the hose lining. Weird to diagnose. Evidence disappears when system is static.

 

[Tugboat]

I just had a few moments this evening to check 2 things
One was some of the fluid removed from the hoses which I was not aware of the shop had retained.
There was a very fine black rubber dust like particles in it ,the hoses are bad.
I also did quick expansion test and they are expanding when the lever is only squeezed about 3/4 pressure
I might be in luck with the new hoses!

 

[grewth]

16mm/ 5/8" sounds about right for dual discs.
I'm not exactly familiar with Honda's set up but my Suzuki GS750's used 14mm and 42mm caliper for single disc, 16mm and 2x38mm for twin discs.
Could you possibly manage to use the braided stainless steel type lines on your bike ?
They have a plastic/nylon pipe inside which resists expansion much better than the rubber hoses.
You might be able to force rubber tubing over the stainless braid to maintain the appearance of rubber hoses