Hello to everyone! Newby here. I recently bought a 27 T Bucket. It had been sitting in a garage for a couple of years and needed some TLC. It has a SBC 350 with a TH 400 Tranny. Got all of that running pretty well. My question is about disc brakes. It came with rear disc brakes only. The calipers are what you would find on 79-85 ElDorado. It had a single reservoir Dorman master cylinder which I believe was for drum brakes. It was not functional. I have replaced both calipers with the same style single piston (2 inch) and have replaced the master cylinder with a dual reservoir with a one inch bore. Both are for disc brakes. The problem is I am still not getting enough stopping power to the brakes. I have a 2 pound valve in the line. The pedal is firm and I have bled the brakes several times just to be sure. I will be adding front brakes if I can find a kit that will work with wire spoke wheels. I am out of ideas and am open to suggestions. Thanks in advance.
Welcome Jim Hewit. From what I am reading with the original single reservoir master cylinder the vehicle did not have working brakes at all(?) Did you trouble shoot the old braking system at all before replacing the parts? Did you replace the metal brake lines and flexable hoses at the same time as the new master cylinder and calipers? Is the master cylinder a corvette clone disc/disc? I am trying to get a better idea of what you have.
When I got the car the original master cylinder piston was frozen. I replaced it with the same thing and I had brakes but not good brakes. I had some doubts about the calipers so I replaced them with new calipers just like what I took off. Still the brakes were not good. I went to the corvette clone disc/disc thinking this will surely give enough PSI to the calipers. It helped but only marginally. To give you some idea of what I have, I have stomped on the brakes on loose gravel and the rear wheels will not lock up. I would think with every thing I have done this would happen. To further answer your question. I have not replaced the flexible hoses or the 3/16 brake lines. I did blow all the lines out with compressed air and when I bleed the lines I get good flow. Hope this gives you a better idea.
Those rear calipers require adjustment. You have to turn the Pistons to take up the slack so that the pads are right on the rotor properly. There's a couple videos on YouTube that show you how to adjust the calipers. Had the same problem with the customers truck that came in with four-wheel disc and the rears were the Eldorado calipers. It had about a 3/16 gap between the rotor and the pads so it took three or four pumps of the pedal to even get a good pedal to stop, once I adjusted the calipers the problems went away good luck.
Dont want to rain on your front spoke parade,but I will.....years ago I had to have a set on my T bucket.looked great. Going around corners I had to slow way down ,or the tires tried to roll off the rims.Even at 1500 lbs the car was too heavy for the tires that would fit. The front disc brakes were suited for go karts,or Harleys. If I did another I would find a spindle that would use true car hubs...early ford, F100, 49-54 Chevy,and run drums, or a disc conversion for same. Then use some sort of solid rim that will take a true car tire.Those cars can be really fun if built correctly.
Wheeltramp Brian. I had the old calipers apart but never realized they were adjustable. Thanks very much for that info. I will be doing that adjustment tomorrow. I have a feeling this will do the trick. I'll let the forum know.
Yes I am finding that out. I have 16 inch wire spoke wheels on the front end. The wheels have four mounting holes for rotors. The calipers on the kits I have seen are small but the mounting brackets must allow the caliper to line up with the rotor. I have not had any issues with tire roll yet. I did replace the existing tires with new ones with tread. With my brake problem, I have not had it out much and I tend to go fairly slow around corners any way. I will keep an eye out for that problem tho. Thanks for the info
Do you have a good "pedal" when you hit the brakes....one stab? Might need a smaller master cylinder diameter to build enough line pressure. Lynn
You are right! My current brake pedal has a ratio of one to 5.75. Both the old master cylinder ( which i believe was for drum brakes had a one inch bore) My new master cylinder has a one inch bore. I also have a 2 pound pressure valve. I have heard from a few people that with these calipers I may need to go to a 10 pound pressure valve. Anyone have any thoughts on that? I am also changing my brake lines to 1/4 inch to help increase the volume going to the calipers. Thanks for your reply
Lynn, I do have good pedal but I have to adjust the brake rod in fairly far to get the pedal to have pressure. I had a friend suggest that going to slightly larger brake lines would increase the volume to each caliper. I will be swapping the existing lines tomorrow. The old lines have many 360 turns in them and the highest point is much higher than my master cylinder. It is mounted under the floor. I will be doing some rerouting and shorting the lines as I go. Thanks for the info, Lynn
Sorry to disagree with the person who said larger lines will increase volume, but they will not. Volume of brake fluid pumped is solely a matter of master cylinder diameter and the stroke of the MC piston. Where a bigger line is necessary to handle more volume is when you're actually pumping fluid from one point to the other. In that case, the size of the line can be increased to reduce pressure drop. High pressure drop can lower the volume coming out of the "far end" of the pipe/tube. But, for increasing the flow via a larger line the "pump" has to be capable of higher volume than is being delivered. In your case, the pump is the master cylinder and the volume is determined, as I said before, by the diameter and stroke of the piston alone. Sorry that's not really answering your question or solving your problem, but at least I feel confident the size of the brake lines has nothing to do with "weak" brakes. Are you sure there's no air in the system? Air compresses and would reduce clamping force on the calipers if the pedal is going all the way down (piston pushed all the way in) when the brakes are applied. Lynn
Increasing your brake line diameter will not affect your system in any way . Fwiw , I'd opine you still have air in the system. Are you bleeding the unused half of the cylinder . where does it pump to ? If you're using one half the mc for one caliper , then the mc is too large . The mc push rod must have free travel or the piston won't return fully.
pedal ratio should be at least 6 to 1. If the total system consists of just two calipers, then my experience says you need a smaller master cylinder . The diameter of the brake lines won't change the volume, only the master cylinder can do that . Disc brakes require more pressure than drum brakes. Smaller master cylinders give less volume but more pressure, More pressure is what you are after. Also the pedal ratio affects both volume & pressure . Got It ?
Slightly larger lines will only increase the volume of fluid in the system. The amount of fluid going to the calipers is controlled by the bore and stroke of the master cylinder. Once the master cylinder piston passes the compensating port, any further travel of the piston takes up slack (shoe to rotor clearance) in the brake assemblies. Once that's done it starts building pressure and should have minimal piston/pedal movement. I don't know what your current pedal movement is like but going to a smaller bore master cylinder will give you increased pressure at the calipers. However, you will have a longer stroke in the pedal. I would think with that car you should easily lock/slide the rear tires. With that said, you should realize that most of the braking is really done with the front brakes which apparently you don't have.
Lynn, this is precisely why i got on this forum. I get good information from experienced people who have been through this stuff. Thanks
Got it, Pete. The master cylinder I am using is the Corvette style. It has two individual reservoirs, one for front brakes and the other for rear brakes. I use this because I intend to install front brakes. Both are for disc brakes. I am using one reservoir for the rear brakes. The other is blocked off . This master cylinder has a 1 inch bore and should provide enough pressure according to the specs. I was not able to bench bleed the master cylinder because it has no bleeder valve installed. I had to bleed it the old fashion way by bleeding the entire system. The old brake lines are a mess. They are 3/16 lines which had many 360 degree turns and high points which lends itself to air in the system. I have repeatedly bleed the system just to be sure. The new lines will be routed differently with as few turns as possible and keeping the high spots to a minimum.
I’ve never seen a MC with a bleeder valve on it? What I do is clamp it in a vice, us those plastic barbed fittings with tubing on them, run them back to the reservoirs. Push in the MC several times till no bubbles are coming through.
Lynn, the bore on this master cylinder is 1 inch. I do not know the stroke, but I will try to research it. I can adjust the push rod and get a good pedal with minimum movement, meaning 1 inch or less before feeling pressure. The more I understand about how the system works the better my chances of getting it working properly. I know very little about the history or build of the car so I am learning as I go. Thanks, Lynn
Hey Budget 36. The first replacement master cylinder as a Dorman 40000. It had a bleeder valve installed which made bench bleeding a snap. The current one has no bleeder valve, I guess I got spoiled with a bleeder valve on the master cylinder
Lake Harley, If I adjust the brake push rod in far enough , maybe an inch, I can get good pedal pressure but the brakes still do not grab. It will eventually stop the car but IMHO it should be much better. I only have rear brakes but with a Corvette style master cylinder and new ElDorado calipers on a 1100 lb car, should stop much better than what I am getting. A panic stop would be a disaster.
Having never dealt with a vehicle using only the rear brakes and a dual reservoir master cylinder - do you have it plumbed to the correct side of the master cylinder? Is the master cylinder built in such a way that it works 70% front /30% rear in its pressure to accommodate the front disc's doing the majority of the work?
Maybe a little slow here but if you plug one half of the mc , where does the air pressure go when you use the other half , they work ( move) together , right ?
I have no idea of how a dual MC could work with brakes on just the rear. Adjusting the MC push rod in 1" would seem to be some compensation for one of the two MC circuits not being used and it might affect the volume/stroke available to the circuit that is being used. It might be acting as if the front brake circuit has a huge leak (it's not even plumbed in) and the other circuit is reacting in "emergency" failure mode. Typically the push rod static adjustment would be a slight amount of clearance between the push rod and the MC piston with the pedal "up", and when the pedal is depressed just slightly MC piston movement begins, which begins to build pressure to the brakes. It sounds to me like you're starting with no clearance at all and actually using up 1" of the available MC piston travel. I'd say unless you add the front brakes you need to remove the dual MC and go back to a simple single MC. Maybe you could use a "T" to hook the two MC circuits together, but that at best would just be a bandaid and adding unnecessary fittings for potential leaks. Lynn
I think the 'good pedal' you're feeling is the unused part of the master cylinder hydraulic locking. A pressure gauge on the rear will tell you the true pressure achieved. I'd be putting a single pot master on it while your not using front brakes. Chris
The Jalopy Journal
