Where is my Brake fluid going?

I've got a 54 Hudson Hornet with a BTV master cylinder. The 4 rubber brake lines along with all the brake cylinders and brakes were replaced. The car now stops on a dime and with the new shocks it is a delight to drive.
My problem is that I seem to be losing brake fluid in the BTV as I need to add fluid every time I take it out for a drive. I find no leaks, but someone suggested that it might be going directly to the motor via the vacuum hose. The vacuum hose goes to a black reservoir tank and then to the engine. I tap the tank and it sounds hollow so I don't think I'm collecting brake fluid into it, but I'm not sure how the inside of that tank is set up. Any suggestions how to find out if my brake fluid is actually passing into this vacuum line and into the motor? Thanks
My problem is that I seem to be losing brake fluid in the BTV as I need to add fluid every time I take it out for a drive. I find no leaks, but someone suggested that it might be going directly to the motor via the vacuum hose. The vacuum hose goes to a black reservoir tank and then to the engine. I tap the tank and it sounds hollow so I don't think I'm collecting brake fluid into it, but I'm not sure how the inside of that tank is set up. Any suggestions how to find out if my brake fluid is actually passing into this vacuum line and into the motor? Thanks

Comments
Hope this helps.
John
Did you try driving with the vacuum hose disconnected and plugged?