Repairing a Brake Proportioning Valve for a Mille GT

When I bought this bike, I noticed that the brake fluid was in poor shape. The brakes worked ok and I thought I would just bleed the system when I got it home. Little did I know what evil lurked in the heart of the Front/ Rear system.

I changed the pads and started bleeding the system. The proportioning valve and the rear brake bled with no problem. Then I couldn’t get fluid to the front brake. It was time to troubleshoot the system. After I checked the front line (for a collapsed brake line), I then checked the Proportioning valve.

The valve has the following connections:

Rear Brake: The rear brake connects out of the bottom of the valve. This is the port marked as POST-R.

Stop Switch: The stop switch is on the next port (labeled STOP) up from the bottom.

Front Brake: The front brake connects in the middle side port labeled as MC-PO.

Master Cylinder: The master cylinder line is connected to the top port (labeled F-ANT).

I took the proportioning valve apart and cleaned it. Once I cleaned it and figured out how it worked I was able to bleed the system and get it working again. The following is a description of what I found inside and how it works.

There is a floating piston inside that is spring loaded in down position when the brakes are off. (Down when mounted on the bike with the rear brake port at the bottom). When you apply the brakes, and the pressure builds in the brake system, the piston moves up. At some brake pressure, (probably related to the Bar reading stamped on the side) the piston covers the front port so no more pressure will go to the front wheel. I assume that is to prevent the front wheel getting locked in a panic stop.

There is one more little trick to this valve. The front brake line connection has a partial check valve built into it. Fluid will travel freely from the left front caliper to the valve and back to the master cylinder. The check valve is fully open when you release the brakes. When you put pressure on from the other side (master cylinder side), the check valve closes and the fluid then goes to the front disk very slowly. It looks like there is a small port in the valve that slows down the speed fluid travels to the front caliper. I guess this is to give the floating piston time to move up and close the port. I did not try to remove this valve as the material holding it is aluminum and I was worried that it would bend.

In my case, the small port had become blocked with crud. I was not getting any fluid through to the front brake. I think when I pushed the front pads away from the disk to remove them, the fluid carried some crud back to the valve and it plugged the small port. Once I removed the valve and cleaned it in brake cleaner, it opened the valve. I then reverse-flushed the front brake with new brake fluid. Everything is now working normal again.

I checked on getting a new valve. It has to be ordered and can take 2 - 4 weeks to arrive. Cost was quoted at $140 to $200. I suggest being careful when you do decide to work on your brake system. I will be replacing the fluid every year from now on to keep it clean and prevent this happening again.

Guzziology lists this valve as being a 4.3/25 ratio, four-way proportional valve, with a part number of 2866 8533, it's only for the Mille GT.

Stan McDaniel