Need a little help please. I converted to front and rear disc brakes using the scarebird conversions and firewall mounted booster with a corvette dual disc brake master cylinder and proportioning valve. The lines are all 3/16. I the scarebird rear conversion call for the 78-80 eldorado calipers with the build in e-brake which I adjusted per directions. The front brakes work correctly but the back brakes don't if I put the car on jack stands and run the engine in drive and step on the brakes I can slow the wheels down but not lock them up. The car will stop but I cannot panic stop it, not safe. The pedal is not soft and stops off of the floor, The vacuum at the booster is fine. For the life of me I can't figure what to try next
You may be filling the front pucks with fluid while braking before the rears get a chance to get working. It would give you a good pedal but no braking from the back. Is everything you have supposed to match up or did you use whatever fit.
those rear calipers can really be a bitch sometimes. give mark a call as i would bet he has a answer. his setups are well engineered to avoid these kinds of issues.
Make sure you have 4 wheel disc prop valve. There is a difference in prop valves depending on whether you have a disc/drum or disc/disc set up.
Everything to do with the calipers & rotor was per the scarebird directions. The power poster, dual disc break master cylinder, and proportioning valve were bought new as a kit off of a chevy web site designed to work together.
Got a pic of your booster/MC assembly? Do you know what size booster? master cyl bore dia? Got a prop valve? Is your ebrake functioning correctly? Sure your pedal travel is sufficient to reach the end of the MC? You're sure there's no leaks in the rear lines? I did a rear disc conversion and didn't realize till the next day I had a leak at the rear axle junction Did you bench bleed the MC? No chance of an obstruction at the hose that goes to the rear axle? No chance there's air in the lines? obvious stuff, but you never know
Oh and you're sure you have the front/rear lines at the correct master outlet or on the prop valve? EDIT: FWIW I just bought a pedal/booster/MC assembly as a unit and the crap aftermarket dual 8" booster broke the rod during brake bleeding, piece of junk, looked OK in a catalog though.
See the answers in red above thats why I am so baffled Thanks for the input. I sent an e-mail to Mark at Scarebird as well asking for help
Here's my experience, I just had a gold colored aftermarket dual 8 " with 1 1/8" bore MC and had 20-25" of vacuum at idle at the booster. I couldn't lock up my brakes either, and after researching I came to the conclusion that the booster just wasn't up to snuff. Well that was before I broke the power booster rod , which turned out to be half plastic when it snapped. Junk. From searching this and other forums(hotrodders.com) I read that the offshore dual 8" doesn't produce nearly as much boost as advertised, and is at best case barely adequate for 4wdb. That seemed to be my experience with it as well. I ended up buying a dual 11" booster with a 1 1/4" bore MC from autozone for $120 , it makes the dual 8" look like a toy. I haven't got it on yet to report back, I've got to do some firewall alteration and it was too late at night to do them the other day If you want one, I've got extra 3000PSI pressure gauges, 1/4" npt, you can attach that in place of the bleeder plugs at the wheels to measure just how many PSI you're getting. Just the gauge though, it's on you to get the fitting to screw it in, I still haven't found a good source. And actually I was talking about the rod between the brake pedal and the power booster, is the pedal bottoming out the master cylinder? is this rod adjustible with the clevis or anything? I had some trouble with that myself, and just 1/4" is really important there. Did you put the MC and the booster together, or were they attached? I can't remember what the installed distance between the two is supposed to be off the top of my head, but I know those little rods in the booster come in different sizes and sometimes need to be modified. here's an article in rod and custom that should tell you a little more, they use a kit to diagnose and have a mention of how they measure that bit Oh so anyway my quick solution, all things considered, would be get a $18 1" bore or maybe 15/16 master cyl from autozone and if everythign else is good that'll give you the pressure that you need. I don't know if the pedal travel will be longer than you like or acceptable, but it's the gotta have it right now thing to try. Good luck, keep us posted! Oh by the way I'm not trying to sound like an expert, I'm just a backyard hack, so take this advice for what its worth
whats wrong with the original 57 drum brakes ? anyone have any bad expireneces with olds brakes ? are you pulling a trailer or do you live on top of a moutain in the desert where your brakes are prone to heating up?
Running 500 plus hp with my 455 and wanted as much stop as I have go wasn't happy with the non self adjusting 11" drums faded real quick plus wanted the dual master cylinder for safety sake
That's a great looking engine, citcapp. I drove a friend's 11 second chevelle with drums, one good stop and those brakes were done. Nobody that drove it would ever think twice about wanting discs! Here's the article that I was trying to find. I really like the methodology used. If you want a pressure gauge that comes with fittings, you can get one from jegs for about $50 here. Even though I have a million of these gauges, I'm tired of looking for fittings so I may just order it myself. Here's another tech article that i thought was pretty useful.
It sounds like your'e pretty knowledgeable about mechanics, but sometimes it's just the basics that jerk you around. First...do you have the correct reservoir on the master going to the correct axle? On a lot of modern master, the front is for the rear brakes, and visa versa. A common mistake, even for people with experience! Next...try bleeding it again. i't very possible your front is bled out, but you might still have air in the rears, that's why they aren't working. The firm pedal is the fronts working. Wne you do a whole car system, it often takes a LONG time to bleed out all the air. A power bleeder really works best in this situation. Manual bleeding is frustrating. othere than that. I'd double check the adjustment on the rear discs. Does the parking brake work? That will tell you if the pads are close enough to the rotors for the hydraulics to work them. Good luck, brake system problems can drive you nuts!
Interesting note about the master cylinder reservoir, I'll look for the original paperwork and see if I have the correct master cylinder, and I'll try bleeding again. First time I've had this much trouble with brakes.
I worked on caddys as a line tech and can remember these calipers were very touchy about the parking brake set-up. The adjustment is critical to brake pedal height. We used to apply tension to the caliper piston while manually working the parking brake lever at the caliper. They are supposed to self adjust while using the parking brakes but self adjusters are supposed to work on drum brakes too. Just my two cents worth.....................................
My 2¢: I seriously doubt that you have any serious problems since you have a solid pedal with good height to it. I'd also count out out any vacuum related issues. The system SHOULD stop the rears even without the vacuum assist. 1. Sanity check on the potential rear/front reservoir mix up. 2. Make sure you have a full stroke at the MC, the fronts stop first, stroke may not be long enough to fully actuate the backs. 3. See what Scarebird suggests, he's hopefully seen everything that could possibly go blooey. Bob
-------------------------------------------------------------------------------------------- I’m sure you have looked at this but, I’ll ask anyway. Is the master cylinder set up for 4 wheel disc? There is check valve in the master cylinder, where the brake lines hook up (Usually a flat “dime like” in shape rubber). As the Brake Pads ware down, this valve is to keep minable hydraulic pressure in the caliper, so the Pads ever so slightly rub the rotor at all time, while the brake pedal is NOT applied. If there is NO check valve for the secondary side of master cylinder, the rear brake pads, over time as you drive, will back off the rotor, causing a gap. With NO check valve, you would have to pump multiple times to get your rear brakes to work. Thus, the overwhelming load the brake system is on the front brake calipers and virtually none on the rear, until you start pumping the brake pedal. I hope this helps. Good luck,
This. Exactly this. I run these calipers and their previous generation on a lot of 4x4 stuff. Put a big adjustable on the parking brake levers on the back of the calipers. Cycle the levers through their travel a whole mess of times. This should move the piston forward and take up the slack. This should help, unless one of the other big things is wrong.
Something else i thought of is the pads, are they clean? A semi-metallic pad is a lot harder than an asbestos pad and may require some burnishing. It is not uncommon for a brand new brake job, pads and rotors not to lock until the pads have been burnished. We had a joke in the dealer as pad material got very hard "Turn the pads and throw out the rotors. Again my two cents so I guess its really my four cents worth. Good Luck
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