i just finished restoring my 67 ss and i put a set of rhoads lifters in there to try them out so i wasnt sure what to expect from the brakes. anyway lately i noticed when i come to a stop and hold my foot on the brakes they will be firm initially then maybe 3 seconds later the pedal will sink about an inch then it will do it another 3 seconds later again and sink just as much and do it to the point of where i cannot stop if i slam the brakes. but if i let off the pedal and push it again i will have firms brakes again until i hold my foot on them. anyone have an idea? i didnt replace anyting in hte brake system yet

 

....yet! Assuming your brake fluid level isn't low, could be an internal leak in the master cylinder, or a leaking wheel cylinder/caliper. Do you have front disk brakes?

 

I agree with Camaro 69. Seen it dozens of times. As he mentioned if there is an internal leak the pressure will bypass the cup seal slowly back into the master, No fluid loss. The pedal just goes slowly to the floor as described. Usually though if you stand on it the stop inside the master will puch on the second piston to give you reduced braking capacity.

 

if there is no external leak, then it will be the master cylinder. a rebuilt one costs about $40 from the parts store since basicly all gm's used the same one forever. bench bleed it first to get fluid through it, then install it and bleed the brakes normally to get all the air out.

 

Not so. If you have a rust area or worn groove on a caliper piston, or a warped rotor making the pistons vibrate in and out, the caliper can take air in on the back-stroke without making it leak. I've had it happen, and it had me chasing my tail because I had no external leaks. I replaced the master cylinder first, assuming that was the problem. But the one caliper that constantly had air when I bled it was the tip-off.

 

everything is stock. no upgrades. 4 wheel drum. ill check in out next time i have a chance. also i should let you know occasionally when i pump the brakes or press real hard quickly one or more of my brakes will lock up. does this help change anyones opinion? it almost feels like all the lines are clogged then break free when i put enough pressure through them. thanks for all the help really appreciate it.

 

Those sound like different problems. Those issues are more likely air in the system or incorrect brake adjustment. You did replace all the flex hoses and metal lines?

 

yes, that does change it. could be sticking wheel cylinders

 

If the adjusters are rusted stiff and can't self-adjust (tighten) the brake shoes like they're supposed to, you'll have too much brake shoe to drum travel, which equates to more pedal travel as well.