Scooterotica

Hi

Just failed my MOT and I need some advice on possible cause and resolution

Issue 1: Binding front brake. Its a ScootRS outboard. I asked the MOT tester for likely cause and he suggested that its often the pistons having some rust on them causing them to stick. What do you think? Are there any other easier fixes I could look at first. If I have to dissemble my brake do I have to drain the hydraulic fluid first. I have semi hydraulic set up with the reservoir under the horncasting - just in case that makes a difference. It was a bitch to set up so I hope to avoid having to do it again.

Issue 2: Steering head bearing has excessive play. Manifested by being able to rock the headstock backwards and forwards a little. Does this just need tightening up or do I potentially need to replace the bearing?

Thanks in advance.

When the nissin caliper on mine decided to self apply, I unbolted it, then jammed one piston, pulled the lever until the other piston eased out a bit, then levered it back. Repeating until nice and free. Swap the jamming devise over and repeat on the other piston. Don't let a piston pop out completely.
I carefully used a squib of ptfe spray under the shitshield. Most sprays will attack your rubbers.

not sure about item 1). Shaun (Lambro) is your man as he's a qualified MOT tester.

Re 2)- if you place your left thumb over the "joint" where the headset and legshields meet and pust the bike forward and then apply the front brake, can you feel movement?- this does indicate slack bearings which is a fail. They are fairly easy to adjust, simply (?) by removing the headset and then tightening the nut and the locknut against it. However......if its too tight then thats a fail as well. Apparently the forks should "fall" easily to one side if the front wheel is off the gound with no apparent play. If you've gone that far you may as well replace the top and bottom bearings with quality items and adjust accordingly. ( you may find that new bearings "bed in " after a while which means adjusting again!!)

I've just replaced mine as I also had some play, and to be honest it probably took an hour or so, to get them "just right" i.e. no play and free fork movement.

Chris

warts wrote:When the nissin caliper on mine decided to self apply, I unbolted it, then jammed one piston, pulled the lever until the other piston eased out a bit, then levered it back. Repeating until nice and free. Swap the jamming devise over and repeat on the other piston. Don't let a piston pop out completely.
I carefully used a squib of ptfe spray under the shitshield. Most sprays will attack your rubbers.

Thanks what did you use to jam the piston? What's the shitshield?

bristolmod wrote:not sure about item 1). Shaun (Lambro) is your man as he's a qualified MOT tester.

Re 2)- if you place your left thumb over the "joint" where the headset and legshields meet and pust the bike forward and then apply the front brake, can you feel movement?- this does indicate slack bearings which is a fail. They are fairly easy to adjust, simply (?) by removing the headset and then tightening the nut and the locknut against it. However......if its too tight then thats a fail as well. Apparently the forks should "fall" easily to one side if the front wheel is off the gound with no apparent play. If you've gone that far you may as well replace the top and bottom bearings with quality items and adjust accordingly. ( you may find that new bearings "bed in " after a while which means adjusting again!!)

I've just replaced mine as I also had some play, and to be honest it probably took an hour or so, to get them "just right" i.e. no play and free fork movement.

Chris

Thanks Chris but it's not the forks rocking from the ring nut and bearing at the top of the forks being free - although possibly the same cause. I've failed for that before and made sure the forks don't move by leaning the scoot over a little and grabbing the forks to see if they will move. The play seems to be only at the top of the headstock.

If the forks are tight with no free play then check the 3 M6 bolts that hold the clamp to the headset. If you've used the scoot over winter there's a good chance the alloy has corroded around the seals making them tight which means pistons out and a good clean up. Also check the caliper backplate is sitting parallel to the disc as the early ones are not rebated around the link peg cutout.

mine failed about 8 years ago for headset bearings never changed them just took it somewhere else

wack 63 wrote:If the forks are tight with no free play then check the 3 M6 bolts that hold the clamp to the headset. If you've used the scoot over winter there's a good chance the alloy has corroded around the seals making them tight which means pistons out and a good clean up. Also check the caliper backplate is sitting parallel to the disc as the early ones are not rebated around the link peg cutout.

If the pistons have to come out does that mean draining the brake fluid first?

Unless you have some way of pumping the pistons out ie compressed air, then you may have to use hydraulic pressure to move them. Take the caliper off the backplate after removing the pad pin screw plugs and slackening the pins, remove the pads and with a suitable spacer ,pump the lever to see if the pistons move freely and evenly and retract slightly. If they don't then you can use the pressure to pop them out evenly.

My stuck piston came free after moving it backwards and forwards a few times. I did use a ptfe/teflon spray. I did not need to remove them completly.
There was enough slack in the flexible brake pipe to allow access by turning the caliper over.
I used a G clamp, but a piece (s) of wood thick enough to stop the first or free piston coming out should do. Then any pressure applied will make the other piston move hopefully. If not then you have more of a problem. An air line at 150psi is not likely to succeed if hydraulic pressure of up to 500psi fails. By then a good soaking may help, but new seals will be needed most likely.
You really need to try strenuously to get both pistons moving like I described above - once you break into the hydraulics you have lost your most powerful means of applying a force to make the pistons come out. As Wack says, work both out a bit at a time, although often once it gets moving you can switch focus to working it to free it off. I had one which worked ok and one seized, so I jammed the free one and concentrated on the stuck one.
On the genuine nissin caliper, the pistons have a rubber ring more or less flush with the caliper body. Its function is to stop water and crud building up in the gap between the piston and caliper where it can cause corrosion and jam the piston. As you have found its functionality is not always 100%.

This is strictly against the rules, but I fill the space behind the s**t shield with hi temp silicone grease. I have never had a problem, even on bikes left outside for years. All the instructions I have read say to assemble the seals dry.
The reason it shouldn't be done is that the seal grips the piston with a carefully calculated force. This causes it to slightly "stick" to the piston. Squeezing the brake lever and a hydraulic force causes the piston to move. This causes the seal to be distorted slightly. Removing the force, ie letting go the brake, and the seal returns to original shape, pulling the piston with it and allowing the pad to pushed back by the disc. Altering the coefficient of friction of the seal and piston with any lube will alter this retraction. This is the theory. Which mostly works.
And of course any type of hydrocarbon based stuff will destroy the all the brake rubbers.
Good Luck
Hope this helps