Proportioner on Hydraulic Brakes?

[daveymc29]

For some of us that bought cars with hydraulic brakes on them reading the trials and tribulations of the mechanicals gives us nightmares. The arms, rods, shafts all have to be perfect not to mention pins and (Pills). Strangely the hydraulics seem to not have nearly the number of failure points but they do have one that can be catastrophic and that's complete fluid loss. (Very rare but possible.) There is one thing that a lot of us lack and that is a proportioner valve to regulate what percentage of braking is done by the front and rear brakes. My question is where did you mount yours and what proportion goes where? Mine work fine, but seems they are equal pressure to all the cylinders which means the size of the bore of the cylinders is what dictates the pressure to the shoes. Been years since I've had mine off yet it seems the fronts were different diameter than the rears. That may make a proportioner a mute point. Any comments? Thanks, not trying to stir up a debate, just curious if I should consider adding to an already adequate system, (39/42 on the front with 43/48 on the rear.)

[Jim/GA]

You are correct. The larger wheel cylinder diameter on the front wheels, compared to rear wheel cylinder diameter, gives you a constant and fixed proportion of hydraulic braking force between the front and the rear. The larger wheel cylinder in front puts more braking force up there (which is what you want). No more proportioning with a valve is needed.

Proportioning valves are used on cars that have disc brakes in the front. Without a PV, the rear wheels are prone to locking up when you brake hard, which results in a skid. The proportioning valve takes a fraction of the braking fluid pressure to the rear wheels off, once the pressure gets above a threshold value.

When I took my Automotive Engineering courses at the Univ. of Michigan (Ann Arbor, west of Detroit) we did a large unit on brake design. I designed several systems as part of that, part of really good course in chassis design.

[daveymc29]

Thanks Jim. Looked at some cylinders I have on a shelf and the fronts are bigger. That means everything else being equal I get a bit more braking force on the front end brakes than the ones on the rear axle. Car has never veered when braking except once when a cylinder had leaked and then it grabbed. My coupe with stock brakes tried to do a loop in the rain recently. I have the brakes,( mechanicals,} off the vehicle at the moment. May opt for Hydraulic's on the coupe also. Have to weigh the cost.

[john in illinois]

Our Model A club had a tour to a brake testing truck safety lane. They allowed us to run our A's through to test the brakes.

I have hydraulic brakes on my Tudor and they tested about 60/40 front to rear and were even left to right. The lane measured all 4 wheel independentlyi.

So you don't need a proportioning valve.

John

[Corley]

To determine need for the valve, just do a few panic stops with someone watching your wheels. If your rear wheels lock up before the fronts, then you should install a proportioning valve in the line to the rear brakes. Best to use an adjustable valve supplied by one of the big hot rod supply houses, that way you can adjust it so all 4 lock up at about the same time. With all stock '40s Ford parts, you will probably not need one on a car, but a CCPU might need the valve since it is so light in the back.

[hardtimes]

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Originally Posted by daveymc29

Thanks Jim. Looked at some cylinders I have on a shelf and the fronts are bigger. That means everything else being equal I get a bit more braking force on the front end brakes than the ones on the rear axle. Car has never veered when braking except once when a cylinder had leaked and then it grabbed. My coupe with stock brakes tried to do a loop in the rain recently. I have the brakes,( mechanicals,} off the vehicle at the moment. May opt for Hydraulic's on the coupe also. Have to weigh the cost.

And there's more to it. Cars need more percentage to the front (we're talking hydraulics here), because more % is needed to front due to more work being done there.

[rocket1]

It is the size of the bore of the master cylinder that determines the pressure going to the wheel cylinders,smaller bore higher pressure.

[Jim Brierley]

last I checked the only cylinder's available for '46-'48 brakes, which are a bit larger than '39-'41. This is not a problem in my experiences. I have had the the earlier style backers on my '31 wide-bed for 50 years and never experienced a problem. I switched to DOT 5 silicone fluid because it doesn't attract water, so the cylinders don't rust. You shouldn't need a PV.

[MikeK]

Down and dirty, if a lockup test shows some need for added bias, you can also play with the hardness/ friction coefficient of the shoe linings. No rule says they have to be the same front & rear.