Background: 66 Lark, originally single resevoir master cylinder/booster with drum brakes. The booster shot craps so my uncle and I replaced it with a "Corvette" style (his words) dual resovoir an 9inch booster which was brand new in the box. It was bench bled and the brakes worked great for 2 years. The Lark has sat for 8yrs since then. I've rebuilt the wheel cylinders, replaced the front brake hoses and bled d@mm near a quarts worth of fluid through the lines. I double checked that all lines and bleeder valves are tight. I also removed the vacuum hose to the booster, inspected it, passing muster, I reinstalled it. The pedal still goes almost to the floor and barely stops the car. I also inspected for leaking lines, none. Any ideas?
Stupid question- did you make sure the rod in the booster is up against the reciever cup on the master? Some corvette style/ bathtub style masters have a deep receiver cup for extra long booster rods. So in other words, you may be pressing the pedal to the floor but the piston in the master might be hardly getting depressed
Try the simplest thing first...bleeding. Bleeding the air in the rear lines can be a real issue. I've gotten air in those lines after replacing components that made me pull my hair out. I learned when bleeding, pump the brakes 3-5 times then hold before opening the valves. This seems to help get those stubborn bubbles out.
Most of the time when you have air in the lines you can quickly pump up the brakes to get a firm pedal up off the floor while a single push firms up at the floor.
When you rebuilt the rear brake cylinders, did you properly adjust the pads for proper drag with the drums?
I thought I did. Adjusted the star wheel until the drums were rubbing on the shoes. I then had a busy step on the pedal and tried turning the drum by hand, if it didn't lock up, I did it some more.
Are you getting a good strong burst of fluid at all 4 wheels when you open the valves when bleeding? If it is just a trickle, it could be air in lines, clogged lines, bad master cylinder, etc. I can't remember if a 66 had the rear rubber hose. Sometimes, those collapse inside when they get old and don't allow fluid to the rear brakes. If the flow is about the same at all wheels, then it could be the master.
I guess I would replace the master, then bleed the crap out of it and see if that helps. Sounds kind of like the wheels are getting fluid but the shoes aren't doing their thing. No oil or glaze on the pads? Had a Nova that did the same thing (4 wheel drum). Ended up being air in the line that took a lot of work to get out. But yeah, it is a stumper.
*sigh* Well, I have been eyeing that new MC Baer came out with. Could the booster have shot craps also?
Dr. Roger: Steel lines on the 66 rear. A collapse of the front driver hose is why I replaced both. ;D BuickV8Mike: I did it right then. I'm glad memory served. (It's weird. I can't always remember things since the cancer and sometimes when I do, I'm not sure if I'm remembering our if I think I'm remembering.)