Brake Master Cylinders Won't Prime-'78 KZ1000 LTD

So I tore down both the front and rear brakes on my bike, and now I can't get either one to "prime" with brake fluid. I did NOT take out the MC pistons, but they both have good push and return suction (when I place my finger over the hole where the banjo bolt goes) Is there something simple I'm missing here? I thought that after bleeding everything dry I could just replace the fluid in the reservoirs and it would pull it through the system.... Obviously you can tell I'm not a brake expert. Any help is appreciated!

jtate1000 wrote:

So I tore down both the front and rear brakes on my bike, and now I can't get either one to "prime" with brake fluid. I did NOT take out the MC pistons, but they both have good push and return suction (when I place my finger over the hole where the banjo bolt goes) Is there something simple I'm missing here? I thought that after bleeding everything dry I could just replace the fluid in the reservoirs and it would pull it through the system.... Obviously you can tell I'm not a brake expert. Any help is appreciated!

When torn down for refurbishing, piston removal is required to inspect the rubber cups and inspect the bore for excessive pitting. Absent piston removal, imo there's not much point to partial dis-assembly of a master cylinder.

Objective of bleeding is to expunge all air from the brake lines.

Be sure the new brake fluid to be inserted into the line hasn't been shaken or dropped so as to contain air bubbles.

When re-filling dry brake system, I usually enjoy good results by using an inexpensive rubber ball syringe with a short length of small clear tubing attached to suck fresh clean fluid into the syringe, hold the tube tip upright, squeeze bulb just enough to expel all air from tube (fluid being at tip of tube).

Then attach tube tip to bleeder nipple, open nipple, and slowly squeeze bulb to fill system. To replenish supply of fluid in bulb, close bleeder, and repeat the foregoing.

With master cylinder reservoir cap removed, should be able to watch it fill as new fluid rises up inside.

Pushing in the fresh air-free fluid expels air inside the system as the fluid fills the system.

Air bubbles in the fluid inside the line are sometimes so tiny as to be almost invisible to the naked eye.

When bleeding the brakes, it's best to have the bleeder screw at 12 o'clock position on the caliper. Mentioned because KZ900-B1 LTD calipers, for example, should be removed and held in held in such position during the bleeding operation, which helps expel all air from the caliper.

Others have different favorite methods and tools. The method here described is cheap and quick, and works for me.

Good Luck!

I had this problem too. You can pump the brakes for 12 straight hours and no fluid will ever be drawn in. If you don't have a mityvac or similar tool, just get some clean hose, attach it to the bleed nipple, open it and manually suck on the hose for a few seconds to draw fluid into the lines.