Couple of MOT fails - advice needed

[Snails]

Thanks for all the advice. I'll get some of the PTFE spray from Halfords on the way home and see how I get on

[johnd]

Make sure you just don't need to file a bit off your brake lever where it pushes on the master cylinder piston. saw a few ScootRS brake set ups that once assembled were pushing on this causing the brake to bind a bit.

[Snails]

Ok I've made a start on the brake binding issue as I guess it will be the hardest to fix

I've bought some ftpe spray and I thought I would start off by removing the calliper and seeing if some spray and a clean would fix the issue. I was reluctant to dis assemble to calliper as I don't really have too much knowledge of how they work and what I can safely take apart with something pinging across the garage.

Anyway I could see the piston were a little gunky some I gave them some ftpe and made sure they were both moving. One thing I don't understand here is what forces the pistons back to their resting position? Having done this I thought it might be worthwhile seeing if there was any improvement from this before going further however when I re fit the calliper its no longer to apply any braking action to the disk and it seems to me that pistons don't come out far enough.

This is a picture showing how far out he pistons will work. I've up the mounting bracket so the pistons are visible.


When its sat properly again the nut will no longer sit inside its rubber seal, not sure if I should be concerned with that or not?


Is it ok to remove the mounting bracket by undoing these two screws? Think that should allow me better access


This shows how much movement there is on the calliper which is why I think is the possible cause of the brake not working?
http://youtu.be/U3LVXhh4Q1Q

[warts]

You need to get that seal back in place, but leave that until you have sorted the pistons.
I'm presuming your pistons are seized. They should move towards the disc, automatially compensating for wear. It looks like yours aren't moving. Has the brake ever worked?
If they won't move under hydraulic pressure then I guess its phukked.
I explained in a previous post how the pistons and seals work to retract.

Did you try what I said in a previous post?
Block one piston so it can't move, then squeeze the brake lever, this should make the other one move. Once it starts, press it back and repeat. Work the piston to and fro until it moves easily, then swop over and do the other one. Avoid pressing too much so the piston comes right out. But it may need a bit of a squeeze to get started.

Once both are freely moving you should then have a brake capable of functioning.

The s**t shield is round the piston and sits in the caliper body. Water and crud gets behind it and jams the piston up. Causing your problem most likely.



Next: The boot you have displaced covers the sliding pin. You noticed there are only pistons on one side? In order to apply pressure to the pads on both sides, the pistons first force the piston side pads against the disc, but the whole caliper body is free to slide away on the pins. Which it does. Until the far pad is drawn into contact with the disc too. Now the disc is trapped and further squeezing increases pressure on both pads & disc and braking starts.
The pins are an essential part of the function. They need to be free to move in the caliper body. With a bit of care you should be able to get the lip inside the rubber into the groove you can just see in the pin. Don't try with anything sharp. It works fine now, but won't last long if water gets in there.
I've no idea if you can get seal overhaul sets.

The spray is PTFE as in PolyTetraFluoroEthylene btw.

[Snails]

The brake was working fine but just sticking a little - I was going for an MOT after all. I may not check everything but I did check the brakes worked first. Both pistons move and I have tried what you suggested about using a g clamp but unfortunately the pads ( I think that's what they are called ) get in the way so if one piston is g clamped the other one can only go so far. How are these pads removed if possible?

[Snails]

Ha I love working from home. Both issues are sorted now.

I was wrong regarding the fork bearings being loose. They were, just not enough to see any movement of the fork legs.

The semi hydraulic mechanism was sticking under my horn casting. Took it off, pushed the piston a couple of times which restored my hydraulic pressure and then refitted horncasting less tightly

It looks like Cinderella will go to the ball after all (well Cleethorpes anyway)

Just need to get that boot back on.

Thanks again for all the help and advice. I learnt a few things about how callipers work anyway.