Had a nasty moment the other day when my front brake lever would not pull in at all - it was rock solid and therefore no brakes- luckily this was just pulling slowly into the cul-de-sac where I live but I don't want to repeat the experience. At first I thought the torpedo shaped bit that the lever pushes had popped out of it's cup due to poor adjustment but when i took the system off it seemed that the master cylinder piston had stuck.
I have taken the master cylinder apart to refit seals but there seems to be no obvious cause so two questions. 1- could their be an other cause i am missing ?- i will look at the caliper anyway.
2. the new seals that nlm have supplied are different to those that came off, the larger one that I presume fits near to the sprinig end is slightly tapered and has a groove - which end should the groove and wider edge of the seal face i presume toward the spring but could be wrong?
thanks - Duncan
brake seals
I recently overhauled the front brake of my Sport.
The seal does fit with the wider end towards the spring - that way it flares open as you apply pressure. Be careful about the free play on the lever: because the piece between the lever & piston is not captive it can jam, or pop out, in extreme circumstances (won't happen if set up correctly).
Also overhaul the caliper. In mine the pistons had become pitted so a friend made up replacements in stainless - won't happen again. I also replaced the wheel bearings at the same time after a warning from the MOT station. When I did this it dawned on me - my bike is 31 years old! It probably had never had wheel bearings replaced or the brakes properly overhauled - you owe it to the bike and yourself.
If you're interested, the Roy Thersby replacement brake disk is a tremendous upgrade. I had previously tried an OEM replacement and it was wrecked by using sintered pads. Roy's version is lighter and better as Cadwell Park proved this year.
The seal does fit with the wider end towards the spring - that way it flares open as you apply pressure. Be careful about the free play on the lever: because the piece between the lever & piston is not captive it can jam, or pop out, in extreme circumstances (won't happen if set up correctly).
Also overhaul the caliper. In mine the pistons had become pitted so a friend made up replacements in stainless - won't happen again. I also replaced the wheel bearings at the same time after a warning from the MOT station. When I did this it dawned on me - my bike is 31 years old! It probably had never had wheel bearings replaced or the brakes properly overhauled - you owe it to the bike and yourself.
If you're interested, the Roy Thersby replacement brake disk is a tremendous upgrade. I had previously tried an OEM replacement and it was wrecked by using sintered pads. Roy's version is lighter and better as Cadwell Park proved this year.
i,ve replaced the seals and stripped the calipers- the master cylinder piston is now sticking slightly as it returns and the brake is proving difficult to bleed so still not right- my question is how exactly to adjust the lever as there seems to be quite a lot of scope and this affects where the seals are in relation to the holes in the master cylinder - at the moment it sems to work best witgh the adjuster screwed in most of the way.
thanks for the help so far - Duncan
thanks for the help so far - Duncan
dunk
Sorry, what I should have added is to get some special rubber grease which is intended for brake systems. NLM sell small satchets for 50p. This should be used on the pistons in the caliper and master cylinder on the surface outside of the seals which doesn't get lubricated by hydraulic fluid.
A sticking master piston needs fixing - it's the sort of thing which lets that loose widget between the lever & piston fall out while you're out riding.
Bleeding: is the caliper mounted in front of the fork? I moved mine behind and had trouble bleeding the brake until I unbolted it and sat in front of the fork with the bleed nipple on top.
How straight is the brake hose? My old hose ran up to a horizontal connector under the bottom yoke (intended for twin discs or a brake light switch) and then up to the master cylinder - that seemed to give trouble in the past when bleeding brakes so I swapped it for a hose straight from the caliper to the master cylinder using angled banjos so there were no horizontal runs.
Having refilled the system with fluid and pumped most of the air out, I turned the bars to the left to raise the master cylinder, walked away and left it overnight. That gave 24 hours for the air in the system to rise to the bleed nipple or back to the master cylinder. The next day a slight slackening of the bleed nipple & a squeeze of the lever got the last bit of air out of the system (air going back up the hose will automatically escape into the master cylinder).
When I overhauled my brakes I also changed the master cylinder to one from a Kanguro (OK, it doesn't look original) which has 66% more leverage & 66% more travel. I was afraid it would just mean the lever coming back to the bars but the system bled properly and I thrashed the bike round Cadwell Park without worries.
Good luck!
A sticking master piston needs fixing - it's the sort of thing which lets that loose widget between the lever & piston fall out while you're out riding.
Bleeding: is the caliper mounted in front of the fork? I moved mine behind and had trouble bleeding the brake until I unbolted it and sat in front of the fork with the bleed nipple on top.
How straight is the brake hose? My old hose ran up to a horizontal connector under the bottom yoke (intended for twin discs or a brake light switch) and then up to the master cylinder - that seemed to give trouble in the past when bleeding brakes so I swapped it for a hose straight from the caliper to the master cylinder using angled banjos so there were no horizontal runs.
Having refilled the system with fluid and pumped most of the air out, I turned the bars to the left to raise the master cylinder, walked away and left it overnight. That gave 24 hours for the air in the system to rise to the bleed nipple or back to the master cylinder. The next day a slight slackening of the bleed nipple & a squeeze of the lever got the last bit of air out of the system (air going back up the hose will automatically escape into the master cylinder).
When I overhauled my brakes I also changed the master cylinder to one from a Kanguro (OK, it doesn't look original) which has 66% more leverage & 66% more travel. I was afraid it would just mean the lever coming back to the bars but the system bled properly and I thrashed the bike round Cadwell Park without worries.
Good luck!