I put a disc brake kit from Speedway on the front of my '39 ford, I think they are chevelle. Then I put a 8.8" rear end off a 1996 ford explorer with disc brakes. I am not crazy about disc brakes on a hot rod, but it was the easy way to go.(or it seemed like it). The car has good, even brakeing, but I can't lock-up the wheels. I am not using a proportioning valve. I installed a 70's corvette master cylinder with 1" pistons. I am considering adding a vacuum brake booster, but am at a lose as to what diameter and if one or dual chambers. any FREE advice on what booster to install ?? Thaks again , Henry
Unless your legs are weaker then mine ( I am legally a cripple) installing a booster will not make your brakes lock up all it does is help you push the peddle. The rear brakes probably need to be adjusted and the master cylinder sized to got with the brakes. Make sure that there is absolutely no air in the system. As far as booster size you are stuck with whatever will fit, and I would probably go with a two stage for peddle feel.
If you have a high, hard pedal you might look at changing the pedal ratio. Typical ratios are in the 6/7 to 1 area, going higher will increase pedal travel but reduce effort.
JFYI, stopping the car at it's maximum deceleration without tire slide is really an ideal situation! If you still want, but cannot attain slide, system pressure is likely the problem, and can be caused by the wrong pedal ratio, and/or lack of physical strength, also possibly poor ergonomics. Do you know about how much effort you are capable of? As already stated, you normally need a 6-7:1 ratio for manual brakes. A booster will provide higher pressure with less pedal efforts. If a booster is needed, use a tandem 7" or 8". Pass on the single 7", as personal experience with these is not good. If you add a booster, you should reduce the pedal ratio to between 3-4:1. An adjustable proportioning valve may be necessary with boosted higher pressures.
take a look at site like mpbrakes.com - need to also figure out if 1" bore on master right for your set up.
Just put the correct sized master cylinder on it...you DON'T NEED a booster. I've got two disc braked cars that can show you... Mike
thanks for the info. Does a smaller piston master cylinder make higher pressure?? If I go from the 1" on it now, will 3/4" give me 25% more line pressure with the same pedal pressure?
I bought the swinging pedal (clutch and brake) from speedway. I thought they had the ratio already figured out. and the corvette master cylinder I am using seems to be popular for hot rods. The way it is now, it takes lots of pedal pressure to stop the car in "normal" driving. (whatever normal driving is for a hotrod). I just get tired of gritting my teeth every time I use the brakes.
I have basically the same Speedway setup with a 7 in dual diaphragm booster and it sucked big time. Went to a mbm disc brake 1 1/8 master and a 8 in duel booster and everything seem to work very well, although I can't lock up rear brakes (which is good in one respect) but they still stop the car. 96 Exploder rear disc brake pads are pretty small and on a heaver car they are adequate , JUST. I bought a pressure gauge and have close to 900lbs pressure with the pedal ratio that came on the Speedway dash mount. I could use a little more vacuum as 14 inches works but 18 would be better
Wanna know a secret? This set up works so nice and fits anywhere. It's easy to make remote fill if you can fab at beginners levels.
You're thinking along the right lines but the maths are I think wrong. I don't think i can do the math properly but i know a swap to 3/4" from 1" is a massive change. The respective areas are 0.44 and 0.78 square inches approx. Going downwards that's a reduction in area of 56% which is a lot. I could be wrong but i think the increase in line pressure with the same input might be 77% more - seems too high to be correct, but i think the numbers go inverse, 1 divided by 0.56 = 1.77. Long time since i was at school and i don't deal with those kind of numbers these days - annoying as i like to think i'm good at that stuff, to a point! Someone help me out! Just redone it another way and allowing for some rounding i don't think i'm far away, but happy to be proven wrong. Chris
Yes, you'll get more pressure (and no, I don't know the math either.. LOL) but pressure isn't the whole story. You also need to displace enough fluid to move the wheel cylinders/caliper pistons so the shoes/pads contact the braking surface with enough pressure. A smaller master will displace less for the same pedal travel, so for the same displacement you'll have more pedal travel. It's possible to run out of pedal travel, so there can be some compromise involved.
The math is simply P = F/A, or pressure out of the master = input force divided by piston/bore area. The smaller the piston/bore size will result in increased pressure output with a given input force, but with a greater travel for the same fluid displacement, as previously stated. You can't get something for nothing! Another formula to keep in mind is F= P x A, where input force equals pressure output X piston/bore area. Input force to the master in the above math is direct force, not pedal force, which will be much less, depending on pedal ratio.
Yep - and a few other applications use the same parts. 94 and earlier is a little different than 95 and later. 13/16" bore
Yes, I admit I was a bit worried until the first test drive. The entire brake system was a hodge Podge of modifications utilizing pieces where space and performance mattered. Lots of math and a whole lots of "well it should work. I'll know for sure when its done" We had a situation where there was a very shitty pedal ratio, no room, small car, huge steering wheel inside the car, too tall of a driver, and seat that couldn't be moved any further. On the fire wall side there used to be room until a V 8 went in the 4 cylinder sized spot. While searching "smallest brake booster" I eventually stumbled my way onto this : http://www.fordfalcon2000.com/restoration/powerbrakes.htm (There's plent more out there too) What caught my attention is that In his review he stated : 2.) I am picky when it comes to brakes. I like a certain feel to them. This setup has a small master cylinder bore, 13/16", compared to the Falcon stock 7/8", or the 1" bore of the Falcon optional power brakes master cylinder. The Geo unit does work, stops the car smoothly and evenly, and locks all four tires in panic stops. The only thing I don't like about them is the feel. It is a "soft" feel. The brakes begin to grab about a 1/4 of the way down, and I can lock them about 5/8 of the way down, but the pedal is too easy to push in my opinion. This "soft" feel, at least for me, had me stopping hard the first couple of times out since I hit the pedal expecting more force would be needed compared to what I truly needed. So I figured his problem of "soft feel" in the pedal could be solved with a pedal ratio adjustment, and I figured he'd need something close to my shitty pedal ratio to get the feel. Since he's giving clues about locking the brakes at 5/8 travel with 1/4 travel free play I figure he's using just 3/8 of his travel or less than 1/2 of it. If his was a 5:1 ratio he could do it with 1/2 or a 3:1 easy. The smaller bore alone takes less foot effort so it feels soft with a normal pedal ratio and softer yet with a booster. However, with a shitty pedal ratio it all feels right and the motion or distance of the pedal pad throw is less for equal clamping force at the wheels. The drawback is the smaller bore M C doesn't displace as much fluid volume so mind your math and caliper piston sizes. The geo metro uses 48 mm pistons up front, the suzuki swift gti uses a 51 mm , 11/16 bore rear wheel cyl or 30mm rear caliper and so if a larger piston is used in the caliper the clamping force goes UP for the same foot pressure but the amount of fluid needed to do it also goes up. The master has a 13/16 bore by 1-1/8 stroke- that's all the displacement you're gonna get out of it. If the caliper piston area is larger the pedal might not feel different but the Clamping force for a given foot pressure increases. The geo has a curb weight of around 2000 lbs The brakes felt great in the pedal, stopped the car flawless, solved the cockpit ergonomics, and fit where there never was a brake booster or master intended. Couldn't have been better in any way.
Thanks for that, it confirms my figures and has done wonders for my mental well-being! Who's a thought a reduction in mc diameter from 1" to 3/4" woulda increased line pressure by 77% !!!! Volume considerations noted. 3/4" with 100lbs under the foot and a 7:1 ratio will give 1584 psi, but the 1" a modest 891psi. I use 13/16" and 6 or 7 :1 with Wilwood type 4 pot calipers and typical late Ford rear drums which all does a great job of stopping hauling a small blocked 32 to a halt. Chris
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