[connman]

are we forgeting about preasure valve needed in rear brake line?

[Mrfixmaster]

Everything new in the back drums, including drums. New master cylinder, okd calipers, new pads on new rotors in front. I adjusted the backs several times to make sure they were right, and bled the system three times. The master cylinder is above both the disc calipers, and the slave cylinders in the back. When I try to stop with any urgency at all, I can lock the fronts up, and get seemingly no action from the backs. When I power brake it, the fronts will lock up, and still spin the rear meats. I have pondered residual valves for front and rear, adjustable proportioning valve, which I have but haven't installed. Stumped, but haven't considered taking it to a S@*P! I can't believe you said that either!!

[Mrfixmaster]

WHAT pressure valve needed in the rear brake lines???? <img src="confused.gif" border="0">

[willys36@aol.com]

Sure sounds like the master cylinder has a bad rear wheel plunger. I would tear it apart, make sure it doesn't have a badly pitted bore (common in used cycliders that have been idle for any time at all), then install a new kit to be sure that isn't the problem. If you have mixed and matched components (if it is a real hot rod, you better have or you loose your man card!), you may have mis-matched cylinder sizes. If the slave cylinders are too small in diameter compared to the master cylinder, you aren't generateing enough force in the rear brakes to establish the friction you need to stop the drums. If you have installed tires that are a lot taller than the brakes were designed for, that will make the brakes seem weak also.

Assuming the master cylinder is in good shape, and if you have a common rear end, go to your parts shop and see if they list larger diameter bore slave cylinders. They usually carry several diameters for common rear ends. Compare the diameters they list to the diameter of yours. If they carry sizes larger than yours, buy a couple of the largest and install them. This should get your rear brakes contributing. Then if they are locking up B4 the front, you can install your pressure regulating valve in the REAR system and all will be well in the universe!

Connman is probably talking about the resudual pressure check valve that should be build into the master cylinder if it was made for drum brakes. The valve maintains about 10psi to precharge the system and prevent air invasion which is required in drum brakes.

[connman]

go look at a lot of 70'-80' gm rear wheel drive cars a lot of them are 3/16 front 1/4 rear residual valve and proportioning valve [adjustible]should complete system make sure bleeders are below mc heigth to bleed

[Mrfixmaster]

I hate to sound like a dumb *****, but if the fronts are still locking up, wouldn't I want the adjustable proportioning valve in the front line to restrict the flow to them? Or do I not understand what you're saying? <img src="confused.gif" border="0">

[willys36@aol.com]

[quote]Originally posted by Mrfixmaster:
<strong>I hate to sound like a dumb *****, but if the fronts are still locking up, wouldn't I want the adjustable proportioning valve in the front line to restrict the flow to them? Or do I not understand what you're saying? </strong><hr></blockquote>

Well, you could do that but the safest approach is to keep the max possible braking ability in front where +60% of the work is done, then adjust the rear to match. I'm afraid that if you got the front to balance with the existing, obviously inadequate, rear, you would not lock up but you probably wouldn't stop either! Best approach is to fix the end that isn't working properly which is the rear in your case.

[ January 17, 2003: Message edited by: willys36@aol.com ]</p>

[hvyeqpmech]

I agree with Willeys36. Having worked on many hydro brake systems his aproach is the way I have found to work.

[Mrfixmaster]

OK NOW I'M REALLY CONFUSED!!! I pulled the master cylinder apart to check for pitting. None there, as I was cleaning it up to re-assemble it I noticed " FT " stamped on the side of the master cylinder above one of the line outlets. Front, you might say. Just as I thought, but when I looked, it is the 1/4" outlet, which I had been using for the back brakes. The other outlet is 3/16, and I have been using it for the fronts. The slave cylinders, and disc calipers are from an 88 Firebird, while the master cylinder is from a non-power brake, full size Chebby.(Thereby retaining my "MAN CARD") I would think that the master cylinder would have larger pistons than the Firebird, and therefore would have more braking power???

Somebody help me stop the madness!!!

<img src="graemlins/pain.gif" border="0" alt="[pain]" /> <img src="graemlins/pain.gif" border="0" alt="[pain]" />

MF

[Chucks,57]

Ok here we go to try to make this as isy as I can Willys36 is telling you right! Im a Truck mechanic (Heavy) and I work on dump trucks and so on and when you lock in youer PTO, and let the motor idel the bed gos up SLOW and when you push up the RPM,s the bed gos up faster you have not changed the amount of Hy oil you have changed the pressher to the ram youer brakes are the samy way the fluid is all ready there when you push the peadel you chang the pressher the the wheel clender remember you are pushing a 1,in disk in the wheel clender not a 20,Ton dump box youer front braks is what stops the car the rear brakes is what keeps the car in a strat line.Sorry for buting in but I thught I might help to cler some of this up.Have a nice day!!

[willys36@aol.com]

I think your problem is unmatched components. Contrary to intuition, your get more braking force with SMALL master cylinder and LARGE slave cylinder diameters. Think of it this way, pressure is "pounds per square inch". the "pounds" in that ratrio is the force you push on the brake pedal - the harder you push, the more "pounds" you place on the master cylinder piston. The "square inch" part of the ratio is the area of the master cylinder. Since this area is divided into the value of the "pounds", a smaller piston gives higher pressure. For 50 pounds of foot pressure (basically a panic stop!)multiplied by the brake pedal arm to 500 pounds, a 1" master cylinder piston has an area of 0.785 square inches which results in 500/0.875 = 570psi in the brake line. For a 7/8" piston, the pressure increases to 831psi. At the other end of the system at the wheel slave cylinder, the pressure is decoded in reverse. Pressure reaching the slave cylinder times area of the piston = pounds of force applied to the brake shoes/pads. Thus a 1" slave piston on drum brakes converts 570psi to 500 pounds force on the shoe push-pin. for the 2 1/2" pistons in disk brakes, the 500psi pressure is multiplied by the much larger area to 2453psi. The difference is necessary since drum brakes "self energize" - basically they bind up inside the drum and give much more stopping friction than you might expect, while disk brakes rely totally on the raw piston pressure to generate the necessary friction.

Things to consider. If your master cylinder comes from a car with power brakes and you are using it manually, the pistons are probably too large. Power brakes rely on the vacuum boost to multiply foot force so can use larger diameter pistons and still get good line pressure. Conversely, manual brakes rely totally on our buff thigh muscle for the pressure so smaller pistons are necessary due to our un-buffness! Usually, the front piston in a duplex master cylinder is dedicated to the front brakes and the rear piston to the rear brakes. Line size doesn't matter since you are moving shuch a small volume of brake fluid. For a 0.25" stroke of a 1" master cylinder piston, you move the grand total of 0.0034 gallons of brake fluid. In fact you are transferring pressure, not fluid.

The way I would attack your problem is first, determine that the foot force you need to apply at the master cylinder is comfortable at lockup of the front brakes. If the foot force seems too little, that becomes dangerous in a panic stop situation so you need large diameter master cylinder piston. If it is too stiff, that might be ok when you take the next step. Get some rear brake slave cylinders with the largest diameter pistons you can find. Now check the braking balance. Pedal force to stop should improve because the rear prakes will contribute more and hopefully the rear brakes will now lock up with the front brakes. If they lock up at the same time with the front slightly before the rears, go have a Miller, your done! If the rears lock up first, install your pressure regulator in the rear brake line and adjust it until the-front-slightly-before-the-rear is achieved.

[Mrfixmaster]

THANKS!! I THINK I'VE FINALLY GOT IT THROUGH MY THICK SKULL! I OWE YOU A MILLER!

MF

[Popeye]

Mrfixmaster Are you stopping yet? Some 'rock solid' braking information can be found on the NTBA website in their forum. Just ask, and a gent that goes by 'Fat Pat' will probably answer. He is a professional that doesn't mind sharing his expertise.

PS Start by connecting your master cylinder up "backwards". Then go to the forum.

<a href="http://www.nationaltbucketalliance.com/ntbabbs/" target="_blank">http://www.nationaltbucketalliance.com/ntbabbs/</a>

-also- If worse comes to worse, e-mail me and we will get it fixed.

[irocz8868]

the size of your lines will not make a differents. 3\16 is fine for the rear considering you only get 20% of your braking from the rear.

[brief madness]

Another possible reason your front brakes are locking up is the adjustment of the play in the push rod from the booster or brake pedal if manual. I had to adjust my new booster push rod into the master cylinder 3 times before it would stop locking up the front brakes. There was some exact measurement involved but found that keeping the very minimum to almost no play was the best. When I mean locking up, I was just touching the pedal and the fronts would lock up.