So guys , my father has a f4 and when he hit the brakes the dashboard lights turn on and the bike starts losing power and turn off . This happens when doesn’t moving
He suspects that it could be a ground that cause the problem.
Any ideas???

 

Sounds like a short in the brake light circult - that should blow the fuse though. Start there - check the fuses, pull the one for the brakes.

Does it matter if it's the lever or pedal? That might help narrow things down.

 

Ground is return-path for power. If there was bad ground, no power would flow and dash wouldn't light up.

So you've got opposite problem, there's a short, meaning power wire for brakes is connected somewhere it shouldn't be.

Here's wiring-diagram for F4. Print it out and trace brake-circuit wires with highlighting marker. Then go to bike and trace actual wires and make sure both ends of each and every wire are connected exactly as shown on diagram.

 

I've outlined pertinent wires on brake-light circuit for you. It's obvious they're getting input power, so trace grn/yel output wires from front & rear switches.

So start at each brake-light switch, find grn/yel output wire, and trace them all way to each rear light-bulb.
They SHOULD NOT connect to anything else. Only light-bulbs.
There's Y-intersection where front & rear brake-switches connect to one wire going to bulbs (OR circuit).
Note on diagram, brake-light output wires don't go anywhere near dash.

 

So guys , he used some wd-40 for electrical contacts on the fuses of the dash and apparently the problem solved . For the moment at least.

 

Excellent news - and now you know what to buy him for Christmas -