WD-40

The only thing that should go into a brake system is the specified brake fluid. Download the workshop manual and follow the procedure in that.

Yeah front break discs are expensive. Sometimes you can get lucky and find a good used set.

There's a nice feature if you click the icon to the left of the start new topic button on the main page. It's called fluid. It puts all the threads from the forums you select in a single list.

Looks a lot simpler now with some of the features turned off

The battery is flat. Give it a charge and see if the bike starts. If it's still struggling to start after it's been charged it needs to be replaced. It probably needs to be replaced if it's dropping to 5V. Cold weather kills weak batteries. You could try push starting it. Turn it on, pull the clutch in, put it in 2nd gear, push it as fast as you can and release the clutch. The bike should start. Pull the clutch in straight away or it'll ride off without you.

On the main page. If you click the icon that's on the left of the start new topic button you get a simple list of threads . It's called "fluid A simpler dynamic listing topics with a forum filter box"

Brake disc bolts can be a b@stard to get out. You should heat the bolts with a blow torch to break down the thread lock. If you don't you risk stripping the threads in the wheel. Use a good quality allen head socket to reduce the risk of rounding the head of the bolt.

Is it not just two screws and the red lens comes off? Number 22 in the last picture https://www.ebay.co.uk/itm/Rear-light-tail-lamp-back-lens-mount-complete-unit-for-Keeway-Superlight-125/222191998548?fits=UKM_Make%3AKeeway%7CUKM_CCM%3A125%7CUKM_Model%3ASuperlight&epid=1668178400&hash=item33bbacd654:g:2QoAAOSwx2dYCew3

Tidy job. Small screen to protect the back of it next?

Ignition fuse blown? Bet he doesn't come back

If you can't find the correct wire in the ignition connector with the multimeter you could push a bit of wire into the back of the neutral switch connector and ground it to the frame. That would mean the ecu gets it's ground through the neutral circuit even when it's in gear and the side stand is down. If it runs now when you put it in gear you know for sure the side stand circuit is the problem and that it's open. Don't ride the bike with the neutral switch grounded, it's just for testing to figure out the fault.

Yes there's a diode in the neutral circuit and one in the side stand circuit. If the one in the side stand circuit was open it could be the problem. No

The bike starts in neutral so the neutral circuit is working. When you take it out of neutral the ground to the ecu comes from the side stand circuit. The side stand circuit is what's at fault. You can confirm it by checking continuity between the ignition switch connector and the side stand connector because on any bike I've had the neutral and side stand circuits join together and then go to the ignition switch and then from the ignition switch to the ecu. You don't have a wiring diagram so you don't know what wire in the ignition switch connector is the correct one to test so you'll have to work it out. You know the neutral circuit is working so use that to work what wire is the correct one. What I'd do it turn the ignition off and put the bike in neutral. With your meter attach the black probe to the neutral switch connector and touch the red probe to each wire in the ignition switch connector. The meter probes have to be connected that way because there's a diode in the circuit. You will have continuity between the neutral switch and one of the wires in the ignition connector. When you find the wire that has continuity keep the probes connected and put the bike in gear. If you're on the correct wire in the ignition connector you will lose continuity when you put the bike in gear. When you find the correct wire leave the red probe in the ignition connector and touch the black probe to the side stand connector. On a good side stand circuit you will have continuity when the side stand is up and no continuity when the side stand is down. If there's an open in the side stand circuit you will see no continuity no matter what position the side stand is in. That will confirm the problem is an open in the side stand circuit between the side stand switch and where it joins to the neutral circuit.

Might not be relevant to your bike but if you google Suzuki Gladius 650 recall there's a few recalls for that model bike. Might be worth checking your one here https://bikes.suzuki.co.uk/owners/check-outstanding-recall/

Two things to check then, check if you've got continuity between one side of the side stand switch and ground. And check if you've got continuity between the opposite side of the side stand switch and one of the terminals at the ignition switch connector. The only possibility that makes sense is there is an open either between the side stand switch and ground or the side stand switch and before where it joins with the neutral switch. Because they join together and your neutral switch circuit is working, so everywhere after that join has to be good. Which isolates the side stand part of that circuit as being faulty. If you want more specific details than that put up the correct wiring diagram.

If you want to do a quick test you could unplug the side stand switch and jump the connector with a bit of wire. That will be the same as the side stand being up all the time. If the bike works with the jumper wire the side stand switch is the fault. If it still doesn't work there's a different fault like a broken wire

I don't have the wiring diagram for your bike so I can't say for sure but typically when the side stand is up, ground goes from the side stand to the ignition switch, then from the ignition switch to the ecu. So if the side stand is up and ignition switch is on there should be a ground at the ecu. If that ground isn't making it back to the ecu, the bike will cut out when it's put in gear because it thinks the side stand is still down. If you can post the correct wiring diagram I should be able to tell you for sure or someone else will.

Is the bike in gear? Is the neutral light on? For the bike to start it has to either be in neutral. Or if it's in gear the side stand has to be up and the clutch lever pulled in.

When you're out chain it to something that can't be moved. If you don't, two people can easily lift it into the back of a van and it will be gone in seconds. Don't trust steering locks or disc locks. Steering locks are easily broken and besides they don't even need to break steering locks or disc locks. They just put something under the front wheel and hold the front brake and then slide the bike away out of sight. Chain it to something that can't be moved

Is the fluid level low? Is the brake pedal moving normally? There's could be air in the system. If there's air in the system when you press the pedal the air compresses instead of the piston moving. Try leaving the brake pedal pressed over night. Sometimes that gets the air back to the reservoir and causes the pedal to firm up. If that doesn't work bleed some new fluid through. If you still can't get any brake pressure, open the bolt at the master cylinder and put your finger over it and press the brake and feel for pressure. If there's no pressure the master cylinder isn't working and will need to be rebuild it. The light staying on is usually because the light switch adjuster is too tight. Slacken it a bit.

What year and exact model is it?

Is it carb or fuel injection? I'd start by checking if the battery terminals are tight. Give the other connectors a once over too, it could be a loose connection. Check if it's got spark when it cuts out. If you've got a good spark but it's not starting it's probably a fuel problem

Could also be water in the fuel. It collects on the bottom of the tank and in the carb bowl so when you go to start it you're trying to start on rain water instead of fuel

I had a gold plated DID chain on my bike when I got it. When the chain was worn out there wasn't a spec of rust on it and the plating was still intact. I replaced it with a regular non plated DID because I wasn't too bothered about the gold look. Regret it now. The outside links definitely rust quicker on the non plated one. The gold plating does help prevent rusting. I will always get the plated chains from now on. Who cares if it shows dirt, it's a chain.

Is there anything under the seat that could be covering the air box inlet?