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Old 09-23-2006, 11:42 AM
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hydralic pressure basics??

The shop maunal on my 41 pontiac claims it has a "proportioned" brake system. 53/47 front bias. It states the front cylinders are 1 inch and the backs are 15/16 ths. Since the system is a single reservoir and the front and rear lines are identical and tee off each other, the only proportioning would be the difference in cylinder sizes. However, this seems backwards to me. With the smaller cylinder diameters in the back, the system should push the rears faster and apply more pressure to the rears, not less. since I have changed the rear end to a 74 Nova, with 7/8ths wheel cylinders the difference is even greater. I'm having trouble with the rear brakes overheating and they appear to be doing most of the stopping, which aligns with my thinking. I think I should put a proportioning valve in the rear system and cut down the pressure back there, but this is contrary to the manual.
Can anyone sort this out for me? thanks,
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Old 09-23-2006, 12:05 PM
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41 pontiac

A smaller cylinder means you have less area for the same amount of pressure.This will determine the force inside the wheel cylinder.
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Old 09-23-2006, 12:13 PM
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A smaller wheel cylinder will exert less pressure on a brake shoe than a larger one will given the same input pressure..

say you have 500 psi working on a piston of 1" diameter.

The area of the 1" piston is .785 sq in. 500 psi acting upon that piston is 392 pounds of force.

a 15/16" piston has .690sq in area. 500 psi acting upon that smaller piston gives 345# of direct force.

A 7/8 wheel cylinder will give 300 pounds of force with 500 psi input.

I would be looking at your brake adjustment and how the fronts are working. If the rears are overheating then they are dragging or the fronts are not working.

Hope this helps, mikey
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Last edited by powerrodsmike; 09-23-2006 at 12:19 PM. Reason: fix my math
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Old 09-23-2006, 12:35 PM
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brake pressure

Pontiac had it right. Think of it this way. Any liquid under pressure will follow the path of least resistance away. The brake springs and brake drums are the resistance. So the pad's should move out first in the rear but with more force in the front. This is what you want so you don't loose control on icy roads and have max braking pressure on front where the most load is. As for the over heating, could they be out of adjustment (dragging)??? You could tell this by taking car up to highway speed for a while than coasting to a stop. If breaks are hot there dragging.
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Old 09-23-2006, 03:02 PM
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brake pressure

Thanks guys. I'm going to have to chew on this. I'm obviously missing something. I've always heard the opposite of the math example.
500 Pounds per square inch, when choked down to .785 square inches (area of the 1 inch circle) increases the pressure to 640 pounds. On the 15/16ths cylinder (.690 sq. in.) it would increase to 710 pounds. Basically, the way a nozzle on a garden hose chokes down the flow and increases pressure. I can't seem to grasp the concept of plugging a 1 inch gareden hose into a 4 inch fire hose to increase the force. Yet, what you say agrees with the manual, so I have to let this soak in, and I'm pretty thick headed. Anyone see the link I'm missing??
In the meantime, I am going to look for rear brake drag, although I had them off last night - everything looked fine and the drums slid on and off without any drag, almost like they should have been tighter. Also did a complete front inspection and readjustment, a little tighter. It stopped better, but it was still cool fronts and hot rear drums.
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Old 09-23-2006, 03:39 PM
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brake problems

It sounds like you have front brake problem is you adjusted them and braking got better. Try thinking of yourself standing on a scale in the back of a pickup. thats xxlbs per sq/ft pressure or weight. Now if there was 2 of you on seperate scales thats twice the square footage, same weight on each = twice the pressure or weight
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Old 09-23-2006, 04:25 PM
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As daoldbuick said, your adjustment or front brake function is probably at fault here.

To help you understand the hydraulics ,you might want to check the following link out about pascal's principle. You are not dealing so much with flow, as you are dealing with pressure in a closed system. the 500 psi example pressure is not choked down, as if it was flowing through a nozzle, in this application pressure is acting upon an area which will move slightly under the pressure. scroll down to the middle of the page on this link.

http://www.lerc.nasa.gov/WWW/K-12/Wi...principle.html

That might help.

later, mikey
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Last edited by powerrodsmike; 09-23-2006 at 04:39 PM.
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Old 09-23-2006, 05:51 PM
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hydraulic pressure

Thanks Mikey, that site was a big help.
Adjusting the front brakes makes the car stop better, but did nothing to reduce the drag/heat buildup in the rears.
I did some tests. Jacking up the car and letting it run in gear without the brakes on caused heat buildup, especially in the right rear. After backing it off and trying again, it was better. Then I shut the car off and spun the wheels. After pumping the brakes several times I tried to spin them again, definetly back to dragging on the right rear. I'm beggining to think I have a wheel cylinder hanging up just enough to keep them dragging. It has new shoes, drum cut, new springs, hold downs etc. But the cylinders seemed OK so I left them. Going to pull the wheels later to be sure it is not a mechanical hangup.
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Old 09-23-2006, 06:17 PM
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Check your wheel cylinders and make sure you dont have a grooved up backing plate. The metal backing on the shoe will wear a groove in the backing plate which can sometimes make the shoes hang up.
A little work with a hand grinder will fix it up.

Later, mikey
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Old 09-25-2006, 09:04 AM
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hydralic pressure

Mikey, right on both counts. Cylinders were sticking a bit and I had some grooves in the backing plates. Just rebuilt it all including the flex hose. Seems fine now. Rears are still warmer than the fronts after a drive. If I understand this right. the 7/8ths cylinders in the rear will engage sooner, but with less force than the fronts. So the backs will get more friction in traffic with light braking, but I need the fronts to lock up first in a panic stop. If I run some skid pad stops and this isn't the case, I assume a proportioning valve is the answer. Even if the fronts lock up first in a panic stop, should I consider a proportioning valve just to balnce it a little closer and get the fronts to pick up a little more use in light brakeing, or is it not worth it?
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Old 09-25-2006, 10:19 AM
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Glad to hear you getting it sorted out.


A proportioning valve can't hurt. I dont know how much it will change the low speed braking, but you DO NOT want the rears to lock up in a panic stop.

I sometimes like to go drive around a little in a dirt parking lot and test the brakes. Any low speed front/ rear bias issues are easily seen on that low traction surface.

later, mikey
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