I'm embarr***ed to post this but I am whipped. I have worked on cars for decades and done lots of brake repairs and bleeding. I have never had a problem until now. I replaced the front calipers on my 49 Buick to fix a problem with it pulling to the right. After replacing the calipers, I could not bleed the brakes and get a good pedal. It never had a great pedal, by the way. The system has a Ford MC (Bendix) with vacuum booster and GM calipers in the front and GM drum brakes in the rear. The MC is mounted below the floor. So, I decided I didn't like way it was plumbed and the ancient proportioning valve was suspect. I replumbed it with new steel lines, residual pressure checks and a hold off valve. (yes, the residual valves are installed correctly.) I also used one of the pump up pressure bleeders. I adjusted the rear drums. Still had very low pedal. I decided that maybe the MC got trashed while I was pumping away on it, so I replaced it. After much pressure bleeding, I went to the old crack the bleeder with a bleeder bottle attached and pumped the brakes from beneath the car. First, I adjusted the pushrod so that there is minimal clearance between it and the new MC. I finally got a few bubbles out of the rear circuit and have a fair pedal now with the engine off. As soon as I apply vacuum, it is mushy and will go almost to the floor. It will stop the car pretty well but it just doesn't feel great. Like I said, it never had great pedal. I suspect the leverage ratio has something to do with this but I am wondering if the vacuum booster is also part of the problem. If anything, I would consider it over boosted. The thing definitely works. Any ideas?
Yes. Fix the above first. If you suspect the leverage ratio, fix it. If you suspect the booster, fix it. Make sure the pedal can fully depress the master, I've found most cannot, in custom applications. Then make sure there are no loops in the lines that could contain air. Lastly, try this when bleeding: Using a bottle and clear line, run the clear line up about 1-2 feet then into the bottle. The air bubbles then will rise so far that they cannot be pulled back into the caliper. And you can see when there is no more air. Cosmo
"Make sure the pedal can fully depress the master, I've found most cannot, in custom applications." I think this is the real problem. I don't think there is any air in the system and the booster works......extremely well.
Was the brakes a kit or was it individual pieces. Reason I ask is that the master could be too large of a bore vs the wheel cylinders and calipers (also GM has metric master/calipers for some years). Pedal ratio could be off, it sounds like the system is working with a low pedal (which you said was always low).
Did you replace the rubber hoses? Could have a hose colaspeing when you apply the brakes, you stated it pulled before and doing brakes and suspension/frame for a living 9 out 10 times a brake pull is always caused by a rubber brake hose and i would guess thats the best place to start.
I have no idea what booster it is. It's about 7 pr 8" in diameter I guess. I will post a picture tonight. I have done the bleeders up and the bench bleed, etc. Also, replaced the hoses. It was not an engineered kit. It was a homemade deal. That why I suspect the lever is not stroking the MC enough. It is a Bendix MC, Ford style for 74 T Bird. Are there larger bore of that style? I need more displacement, not less.
Did you use any of those through the frame connectors the ones p*** through tubing frames some are almost impossible to bleed.
Are you using DOT 3 or 4 brake fluid or DOT 5 silicone? If Dot 5 silicone, air bubbles in the fluid are very hard to get out. Also, is your master cylinder below the level of the caliper bleeders?
It appears I have a 7" dual diaphragm booster. Also, the pedal has a 12" to 2" ratio. Given about 7" of travel to the floor, I estimate I have a maximum of 1 1/8" of travel at the MC. Unfortunately, I did not measure the travel before I installed it and gave up the core at the auto parts store. It is from a 74 T-Bird but it is a standard Bendix MC of that era. Anyone have any idea how much the total stroke of this MC is?
The '74 T-bird master has a 1" bore. The stroke is probably closer to 1 3/8", based on Ford masters I have measured. Your pedal ratio is too low for power- it should be around 3:1 instead of your 6:1. The booster push rod to master clearance is typically .040", and that's checked with full vacuum (about 20" Hg) in the booster. The master has to be able to be fully stroked before the pedal runs out of travel, period. This should be checked by opening both front/rear systems and pushing the pedal. You should hear/feel the master bottom out. No vacuum ***ist is necessary during this check.
Thanks, Bob. There is no way this master is bottoming out and I did not know to check the clearance with full vacuum on the booster. Both of these will help. Thanks again!
Lengthened my secondary lever to a 4:1 ratio and guess what! I now have great brake pedal. If you convert a car over to piwer brakes and use the old non power pedals, you had better check to see if you are getting enough travel on the MC rod. I'm betting that some of the problems that HAMB'ers are having with "bleeding" their brakes is really a stroke problem. It was for me. Thanks again, Bob!
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