'91 K75S Brake bleeding not going well

Can someone tell me what I am doing wrong on this job? 91 K75S. I replaced the handlebar master cylinder with a new one about a year ago. I believe it worked fine with the old lines, though it's been a while and the bike has just been sitting. The top line has been off for a some time but I put the bolt back in the cylinder when I removed it, and there has always been fluid in the reservoir.

I have now replaced all three lines with Spieglers. I am using a Mighty bleeder at the calipers. I've gone back and forth to each side, sucking out the fluid. I've pulled about 1 1/2 cups of fluid out. I get no resistance at the handle. Although every time I pull the handle, air bubbles come up in the reservoir. The manual makes it seem so easy, and I've bled a few brake systems before. What should I be doing to get the brakes to work, that I'm not doing? Thanks.

Patience you must have.

On K75 front brakes I prefer to sometimes just let gravity do the work. Keep the reservoir full and let gravity pull the fluid down.

Is ABS an issue on your model? There is a Similar Thread in the pane below but it's for a K1200LT and seems ABS needed to be considered in bleeding.

If not, do what Paul said!

IMHO, skip the MityVac and bleed the conventional way, pushing fluid through the system from the master cylinder. But first, crack loose the line at the master cylinder and bleed the master cylinder. Once that’s done, continue on down the system bleeding as you go.

Best,
DeVern

John,

If you look at the master cylinder with the handle bars pointed straight ahead,you will notice that the rear end (lever end) of the bore is lower than the front end (where the hose exits). The hose exits from the bottom of the bore. This traps an air pocket at the front upper end of the bore, just above where the hose exits.

To bleed this area, you need to turn the bars all the way to the left. This will put the rear of the master bore (where the reservoir bleed back hole is) at the highest point in the system. You can then use a vacuum bleeder.

Another thing to be aware of when bleeding brakes manually; Over time, corrosion develops inside the bore of a master cylinder between the area where the piston NORMALLY travels and the end of the bore. When you manually bleed the system by pulling the lever all the way to the grip, you are causing the piston seals to travel over this corrosion. This can damage the seals. This can cause premature failure.

To safely manually bleed the brakes, tape a block of wood to the grip that stops the lever from traveling further than it travels under normal braking. (Yes, you need to remove the block when done bleeding )

Thanks for the help. I tried the method of turning the bars to the left, and sure enough, after a few pumps of the handle it firmed right up. I think that job is done.

One thing that hasn't been mentioned yet (but is in other threads I'm sure), is to tie back the handle overnight, then bleed the brake system one final time. As I understand it, the pressure in the system forces any remaining air bubbles into the brake fluid, eliminating them with the final bleed. This obviously only works when you have the ability to build pressure.

drneo66 said:

One thing that hasn't been mentioned yet (but is in other threads I'm sure), is to tie back the handle overnight, then bleed the brake system one final time. As I understand it, the pressure in the system forces any remaining air bubbles into the brake fluid, eliminating them with the final bleed. This obviously only works when you have the ability to build pressure.

Click to expand...

Thanks, I may give that a try, just to make sure all the air is out. The handle pulls back about half way right now, but then firms up pretty solid. Maybe that is normal.