I've read every ATV/Yamaha board and have tried so many things, I'm stumped. Problem started the front brakes would lock up when hot, loosen the bleed valve they would unlock. On many different people's advise, bought a used master cylinder and rebuilt it, changed the brake pads and all the brake lines and then greased the caliper pistons and pins. Now I can't get the brakes to work at all.
Prior to reassembly, I verified all the lines were clean, even shot a little compressed air down each line after the distribution block?? and each caliper compressed and then I was able to push the piston back in.
Tried to bleed the system using both the service manual way and a way found on another form and I get nothing, no fluid comes from the bleeder valve. However, at first I didn't have the bottom union bolts tight on the calipers and they were leaking brake fluid. I also filled the master cylinder through the union bolt hole and then attached it to the handle bar hoping that would help, it changed nothing. Is it possible to have a bad union bolt?
Before I spend $180 on new calipers and still have the issue, does anyone have any other ideas? Thanks in advance.

 

Does the used master cylinder build up pressure? Take the main line off and block the opening with your thumb and an old towel.If it doesn't build up pressure,may need a new piston/seal kit if available.It has to build up pressure or you're just spinning your wheels on bleeding the brake system.Don't know if you've used a Mitvac,but they usually help along with the air assisted bleeders if the master cylinder is okay.

 

If I block the master cylinder port, the handle has pressure. I'll have to look into the mitvac, thanks.

 

Not that expensive at Harbor freight,etc.Brake Bleeder and Vacuum Pump Kit

 

Quad front brakes often refuse to bleed, I suspect it is due to the small size of the master cylinder. The following is regarded as bad practice but it works. Clamp a pair of vice grips on the rubber pipe from the master cylinder so that it stops the flow of fluid, you will get the lever locked solid if you have got it right. Remove the grips, open a bleed valve and squeeze the lever down, refit the grips and let the lever return, repeat until the system is bled, finishing by closing the bleed valve with the lever down as normal.