71 gmc brake light on and leaky brake m.s. out of cap

[gmc1971harwich]

i was wondering if u guys can help. i have a 71 gmc p-up lately my brake pedal has been really spongy so i checked all the lines and found that the rear line on the drivers side has a small leak. i ran it out of brake fluid and the brake light came on. when i filled the reservoir back up it kept leaking out of the top of the cap oozing out when i hit the brakes. i think i might have overfilled it. how far should i fill the master cylinder? i didnt see any kind of full mark under the cap. and how do i get the brake light off on the dash. also i think i heard my proportioning valve click over when i pulled in the driveway last how do i know that its not stuck one way or the other? i have a 2wd 3/4 ton truck with discs in the front and drums in the back. since i have filled the m.c. brakes have felt fine didnt bleed yet but i know i have to bleed them after i replace the line this weekend