brakes not again

[chevy6673]

my 73vette did this to me 1 time last year and now again the brake light goes on when i step on brakes so i opened the mastercylinder and again the back reservoir is empty no biggie i took back wheels off and looked for leak but there is none so i full up back reservoir again and have someone step on brakes still do not see leaks and nothing dripping from lines ,that i can see and the reservoir is still full ,whats next thanks 4 any help

 

[JohnZ]

my 73vette did this to me 1 time last year and now again the brake light goes on when i step on brakes so i opened the mastercylinder and again the back reservoir is empty no biggie i took back wheels off and looked for leak but there is none so i full up back reservoir again and have someone step on brakes still do not see leaks and nothing dripping from lines ,that i can see and the reservoir is still full ,whats next thanks 4 any help

If it isn't leaking from the lines or calipers, it's probably leaking out the rear of the master cylinder, into the booster; if you have manual brakes, it's probably soaked the dash mat inside the car (up where the pedal support attaches to the firewall) with brake fluid.