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

[cdowns]

small leaks have a tendency to become large leaks quickly and suddenly

to try putting light out forcefully stomp on the brakes this sometimes works

best is to fix your brake problem then bleed your brakes according to directions in factory service manual

as far as leak rom the cap take off cap and clean mating surfaces real good and try again

with the leak the light will contiue to come on frequently till that half of the brakes fail completely

[dznucks]

Like cdowns said, check your mc cap for crud or grime, but also look for any type of damage to the rubber seal. Replace if neccesary.

you obviously dont want to fill the MC up so full that it overflows, but i dont think there is "full", just keep it topped off so air doesnt get into the system.

Also check to make sure the metal retainers that keep the cap on, are good and tight.

The proportioning valve should reset itself once you have fixed all your issues and the pressure is equalized between your front and back brake lines. you can slam the brakes like cdowns said or use a BFH to hit the brake pedal. there is also a tool out there you can buy that resets it as well.

[BentHeadBolt]

Go easy on the BFH. Strong continuous foot pressure should work if the lines do not leak and are free of floating dirt or any other line obstruction.

[gmc1971harwich]

there was caked on mud colored crud in the master cylinder is it rust or just old crappy brake fluid? i got most of it but there was some i couldnt get out, should i replace the m.s. because of this?

[gmc1971harwich]

the seal under the cover looks ok, no cracks anywhere