Hey guys... My T was originally equipped with 4 wheel disc brakes. WS6 Trans Am on the rear with big, midsize calipers and rotors up front. The master is a 4 wheel disc, 1" bore non-power unit from a second gen Trans Am. Anyway...in use that previous setup was back flowing and you had to give the pedal 2 or 3 pumps to get the brake pedal high and in addition, the handbrake was gonna be a bitch to hook up to those silly rear calipers. To solve the problems I removed the rear discs and replaced them with S10 drum assemblies. The front discs remained as they were. I also replumbed the master to use a 2 lb RPV up front and a 10 lb to the rear drums. Got a good pedal now and not a whole lot of pedal effort to stop. THE PROBLEM: I've noticed that the front brakes seem to be doing the lions share of braking. When I stop on gravel etc its very easy to lock the front wheels while the rears roll along. During the initial conversion, I went to Parts U Pick to get a test drum and in the process actually found some really good drums and like new internals so I bought them and ran them. Just added some new shoes and deglazed the used drums. Now...by doing this I ended up using the S10 wheel cylinders and they have a small, 3/4" bore! My thinking is that I should have at least 7/8" or 15/16" cylinders to increase clamping pressure to the rear wheels. Am I on the right track with this? I tried the 3/4" cylinders because they were new to the vehicle I got the assemblies from, but I have no issues with buying more at a bigger size if thats the problem. thoughts???
It would definitely help to put in the biggest wheel cylinders and a proportioning vale to dial the rear brakes in..
Maybe try a hold off vavle for the front. Keeps the front discs from actuating until the rear drums catch first. Speedway sells them for disc/drum setups. Bigger rear wheel cylinders would also be on my list, the 15/16" size.
Seems like everyone is on the same page with going bigger on the wheel cylinders. I think things are pretty close so I will make one change at a time to see where it takes me. Hold off valve...thats a new one on me! I gotta check that out just to see how it works!
Are the rear shoes adjustable and too far from the drums? The fronts should always lock before the rear or the rear of the cart may be swung to the front in a fast stop if the rears lock first.
I think you were on the right track before. However, I am not so sure you even have a problem. The front brakes normally do the lion's share of braking for two reasons. One is weight transfer during a stop...and the other is vehicle stability when braking hard. The LAST thing you want is the rear brakes more effective (able to lock up first) than the fronts. Nothing will cause your vehicle to swap ends quicker than rear brake lock up before the fronts. You mention having been on gravel while noticing the brake characteristics. I would find an empty paved parking lot and make some braking tests, especially a 'panic' stop. If, after that you still think you need more rear brake effective ness, and you may, I would try the larger rear wheel cylinders. I would not screw around with a proportioning valve from the git go. It may very well not be needed. You mention your "T"..... If it is a vehicle whose operating weight varies little when you drive it, unlike a family truckster, you should be able to dial in a good brake balance by the method you propose. You seem to have a good grip on how this stuff works. Ray
Put S10 callipers on the front to get the correct balance with S10 rear brakes. [ and S10 discs to get matching brake torque ] With single and tandem master cylinders the line pressure is the same throughout the plumbing system. It is when this line pressure is converted to clamping pressure [ using fluid mechanics ] that the balance occurs. Factories use different swept volume in the callipers and rear slaves to get the correct balance. They have done the hard work for you [ unless you're good at math ] Different M/C sizes only alter pedal travel/effort and line pressure. A proportioning valve is a patch up method that people use when they "Get out of their depth"
Sooo, all those brake engineers from Bendix, GM, Mopar, KH etc. must have been "out of their depth" when they designed proportioning into production disc/drum and disc/disc systems. Learn something every day.
No, but it sounds like the average F***wit is out of their depth . Those proportioning valves you are referring to , are actually pressure limiting "regulators" When they get to a maximum pressure or higher, they don't allow that higher pressure to pass through. They do not proportion the fluids at lower pressures. This is what Fluid mechanics do. Anybody that uses a proportioning valve to fix their mismatched cowshed engineering is a real F***wit, that should stay away from cars. If you had a poorly balanced brake system with a proportioning valve, and the traction conditions were less . You would be in a dangerous situation because the brakes [at one end] would lock up before the maximum pressures were obtained. A true proportioning system is Twin master cylinders with a balance bar.
Since I'm a "F-wit" I'll not tell you how a proportioning valve REALLY works....""A true proportioning system is Twin master cylinders with a balance bar."" Yes after you adjust the balance bar...
Bruskie...I doubt the S10 valve will be of any help. The front calipers are much larger than s10 calipers and that will throw a monkey wrench into its ability to do its job properly. Loudbang...the brakes are properly adjusted. I immeadiately checked that once I realized that there was a potential issue...and while they did adjust a small bit due to the new shoes bedding in...it had no noticeable effect on the rear brake effectiveness. Hnstray...I feel you are dead on when you say you want the fronts to do a greater portion of the braking...but my butt is telling me they are doing a little too much! LoL I'm weary of doing full on panic stops on pavement with my small diameter hairpins. They have been there for years but don't have any tie braces between the upper and lower bars. I'm not fussy on that and will be adding them over the winter to increase strength. Until then I will save the panic stops for times of real panic! Kerrynzl...I don't want to change the fronts as they work great with nice pedal feel...not to mention that changing the calipers will require changing the mounting brackets and potentially the master cylinder. I think its a lot simpler and more likely to keep that good pedal feel to go with a change in the rear. I have to see what the big mid-size Chevys were running back in the mid 70's and see if something of similar size will match up to the S10 backing plates. Who knows...maybe the mid-size wheel cylinders are a direct bolt on! Regarding the bantering back and forth over use of Proportioning valves. Its pretty much a moot point as I don't think adding a proportioning valve will help my issue in any way. I need an increase in braking effectiveness to the rear...not a way to limit it. Guys...I REALLY thank you for having this little discussion. It makes me feel like I'm on the right track with this. This Hot Rod stuff always involves experimentation but its definately nice to hear from others who have already gone thru the same basic steps!
I hope you will post the steps you take and the results you achieve. This is an area of particular interest to me and I tend to agree, in principle, with kerrynzl about the many mismatched systems and the 'magic cure all' proportioning valve. A few years ago I set up my then project '47 Ford Coupe with a 12" front disc brake kit with '71/'76 GM full size car calipers and a Ford 8" with 10" x 1.75" (as I recall) drums, a '67/'72 Mustang master cylinder on the stock '47 pedal, 2# frt, 10# rear RPVs, no proportioning valve and no booster. I got moderate pedal pressure with great brake modulation and what certainly seemed like very good balance. Again, I think you know your stuff......let us know the outcome. Ray
Do you actually know what the front callipers are from? If you don't want to change them , then you need to match the rears [as you were already suggesting ] You might need larger drums to increase rear brake torque. DO NOT put a proportioning valve on the front system, these only limit the "increase in pressure" beyond a pre-set maximum pressure [ this can have adjustable spring pressure ] Manufacturers use these proportioning valves on the rear to prevent lockup under extreme weight transfer and braking. They normally have more rear bias dialled into the car for normal braking conditions or mild braking [wet weather etc]. If you removed the proportioning valve from a car, with normal driving you wouldn't even notice the difference, UNTIL you jump hard on the brakes which causes the rears to lock. "My choice" would be a twin master cylinder "pedal box" , this way you can match the brake bias with individual master cylinders for front and rear [for swept volume] Then you can also adjust the balance further. Try and improve the braking system, not patch it.
I don't know the exact model car...but mid-size Chevy from the 70's. Larger than the usual "metric" calipers. You may be on to something that I wasn't really considering...drum diameter. Interesting. Hopefully this won't be the case, but regardless it's still worth the trouble to swap in some larger cylinders as they will also work with larger diameter drums if they turn out to be needed. No prop valve anywhere preferably, but never on the front regardless! Tonight I will do some checking to find out what size drum usually matched to those style rotors/calipers. Thanks Kerry!
Your setup sounds almost identical to the brakes in my roadster. I have the big GM calipers on front, S10 drums on the rear, and a 1" master. The only difference is that I have a Speedway proportioning valve plumbed in. The front brakes do most of the work like yours. You really don't want rears to lock up before the front; swapping . ends in the rain is no fun
Don't forget to take in the size of the rear tires compared to OEM cars. OEM cars usually have the same tires on all 4 wheels. So matching front to rear could take some experimenting. Ago
I don't want to "burst your bubble" here, but in road racing we dial in more rear brake bias in the rain. Why, because there is less traction so there is less deceleration and less weight transfer to the front. [our braking is considered to be on the conservative side of having an accident] When thinking about brakes, you need to start from the tyre footprint and work backwards to the drivers foot. OEM cars have the brake balance more to the rear, and a proportioning valve in the rear system to stop rear lockup under extreme conditions "Ago" is correct about tyre sizes With the Big and Little situation of most hot rods, would point to more rear brake bias
There is so much incorrect information in this thread. If you want to REALLY learn about braking systems the "Brake Handbook" by Fred Puhn is a good place to start.
The Jalopy Journal
