CB750 F1 Rear Brake Bleed

[Priddy]

 Anyone come across problems bleeding out the rear brake? after a master cylinder rebuild (checked and checked again!) I can get nothing, do you need a vacum bleeder?

[K2-K6]

Any system should go through a normal bleed without much assistance but if it's very resistant it usually means a bleed of air hole is blocked.

Try taking the hose off the caliper and hold it over a bowl, then bleed it through the pipe only to see if the problem is with the master cylinder.

Was it working before the rebuild?

Even a vent hole to prevent a vacuum forming when the lid is on the master can stop you getting the fluid to go down to the caliper so have you tryed it with the lid off during bleading?

[Priddy]

Thanks for your reply, I have tried with the lid off and even taken the pipe off the back now but seems not to be pumping. All holes clear and every thing new so i am going back to the drawing board tomorow - i am obviously missing something!

[K2-K6]

It's usually something blocking a small hole, so a really good close look through all of the master cylinder should turn up something for you.

Apologies for the typos in my last post as it may have seemed a  bit odd.

Most systems almost let the fluid run through when you open the blead nipple as the pathway is essentially open right through when the lever is not depressed.

Hope you get some progress.

[Bryanj]

Most japanese master cylinders I have worked on do not like pumping after a rebuild.

After lots of different trys the method i found that worked is this:-

Disconnect the outlet pipe

Cover everything in the area so any fluid will not get to anything it can damage

WEARING A RUBBER/NYLON GLOVE put a finger over the outlet hole

With the master reservoir filled push down on/pull in the lever

Lift off then replace the finger

Let off the pedal/lever and repeat from push down/pull in


Eventually the master will pressurise and try to blow your finger off the hole, squirting fluid everywhere---hence the covering up.

At this point you need to hold the lever down/in and connect the pipe and bleed to the caliper nipple.

This metod may take 10 mins + but i have never had it fail yet

[Priddy]

Many thanks for the advice, after a long process I eventually decided to connect an ezibleed system to the caliper via the bleed nipple to 'prime' the braking system in an effort to either fix the problem or highlight an issue. When priming under pressure (pumping fluid in from the caliper end) I noted a hiss from the brake hose and found a new hose leaking thus preventing bleeding! put the old hose on and bled up straight away.
Thanks again all for the advice! much appreciated.

[K2-K6]

Oooh, wouldn't have guessed that one. At least you have got it sorted now though.

I know that people do change the brake pipes on old bikes as a matter of routine, the original spec is quite good though and I've not seen one fail even as they age on many old Hondas.

[Waggles]

As a matter of interest I changed mine for braided HEL lines for my rebuild as the originals looked old and 'crazed'. I investigated further by cutting the old ones and I seriously doubt they were in any way impaired, very well made as you say. Not sure what an MOT man would say though.

I think the braided lines give a slightly firmer feel but its not dramatic, would be happy to replace mine with standard if they needed it.

[Bryanj]

There is nothing in black and white in the testers manual, but when I did my course it was anything deeper than a surface crack in the outer should be failed with an unwritten comment that If in any doubt fail it as it is a safety related item

[Green1]

I had a similar problem with googridge hoses on a ford puma air kept leaking into the system so ended up putting the standard ones back on
I have never noticed any difference in performance with braided hoses so I don,t bother with them anymore.