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is there anything behind the 2 brass seats that can get damaged if you decide to run a solvent threw them to make sure there clean? btw its a 1 1/8" PDB master. is it like the drum brake master that has a resdual check valve behind the brass seats?
 

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It is possible the rear port going to the rear brakes has a residual check valve. That is if the master cylinder was set up for disc/drum combo. The check valve is there just to keep a little brake pressure on the cups in the wheel cylinders. Somewhere around 5 to 10 psi. Usually not found on front when using disc brakes. Only way to know for sure is to pull the seat out or maybe use a small flashlight and look through the seat. You may also very lightly insert a small piece of wire or toothpick through seat and feel for a check valve. Careful not to damage it. Its a rubber duck bill style on the originals. Also you can apply a little shop air, less than 10 psi through seat to see if it holds pressure. If it blows straight through, it most likely does not have a check valve. Solvent may be harsh on those rubber check valves, if it does have one installed.
 

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It is possible the rear port going to the rear brakes has a residual check valve. That is if the master cylinder was set up for disc/drum combo. The check valve is there just to keep a little brake pressure on the cups in the wheel cylinders. Somewhere around 5 to 10 psi. Usually not found on front when using disc brakes. Only way to know for sure is to pull the seat out or maybe use a small flashlight and look through the seat. You may also very lightly insert a small piece of wire or toothpick through seat and feel for a check valve. Careful not to damage it. Its a rubber duck bill style on the originals. Also you can apply a little shop air, less than 10 psi through seat to see if it holds pressure. If it blows straight through, it most likely does not have a check valve. Solvent may be harsh on those rubber check valves, if it does have one installed.
is this what the rubber valves look like?
 

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