Might be. Those proportioning valves have a shuttle cock inside that acts as a trigger for the brake light sensor. It also stops fluid flow if there is a failure in the brakes, say a rear wheel cylinder fails, the loss of pressure forces the valve to close as to the rear brakes. That may be the fluid out of the cap or it could just be as HRL noted, that you are overfilling the master.
Just as an fyi on the proportioning valve, one of the issues is that to bleed the brakes, you actually have to remove the brake light switch and insert a small tool that keeps the brass shuttle cock centered. otherwise when you crack open a cylinder or caliper, the loss of pressure moves the shuttle cock and you have to then get it re centered. I just went through this same thing with a ford truck that had essentially a chevy brake system on it. We discovered the rear brakes were not working and the shoes were in such good condition it looked like they had never worked, although the owner, the uncle of a friend of mine, swore up and down the truck stopped fine. It ended up the valve had tripped and was in the tripped position so long it was now simply corroded in place. The rear brakes had not in fact been working at all. Replaced the valve and properly bled the brakes and the truck stopped like is should.