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Tips

BN7 Brake master cylinder

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Norman

Norman

Freshman Member
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Any advice welcome - I can't put off changing the brake master cylinder any longer. Looks pretty straightforward but comments from anyone who has done this would be most welcome. Are there any pitfalls ? Do I actually need to bleed the brakes after - I won't be disturbing any of the fluid in the actual brake lines, so will any air in the new master cylinder just rise to the surface and escape after filling, or is this just wishful thinking?
 
Unfortunately, when you remove the brake lines from the old MC you will introduce air into the system. The good thing is bleeding brakes is an easy job (especially with a pressure bleeder) and you will have peace of mind that the fluid in the system is all new. Might consider bleeding the clutch slave too so that the reservoir will contain all new fluid.

Cheers,
John
 
I recently replaced my master cylinders for both brake and clutch. Quality of the aftermarket cylinders is somewhat inferior to the originals. You might consider re-sleeving rather than replacing. Removal & install is fairly straightforward... and yes, you will need to bleed the system when you are finished with the job.
 
I have found that getting the master cylinders in place and connecting the lines is an absolute bear with the carbs installed. But it can be done. I have done it both ways. You will need to bleed the system because the air you introduce is sort of mid system. it will have to be pushed all the way thru to the wheel bleeders unless you can reverse bleed which is a more sophisicated approach that most hobbiest can't do. Pressure bleeding like John said is the best way that a hobbiest can do. If you have to bleed it by 'pump the pedal method' it can be laborious and frustrating, because IMO the volume of the master cylinder is not near the volume of the caliper bores and the master cylinder piston does not displace much fluid with each stroke. With the system bled up well, the master cyl. piston doesn't have to displace much to make the brakes work, but when trying to move all the fluid thru the lines the master isn't quite up to it. So a pressure bleed is better.
 
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