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Old 08-14-2007, 08:51 AM
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Default Residual Valve Placement

Hey guys,

I am going to replace my MC. The Commando has front discs, rear drums. Since the brakes worked when I got it I assume that it has the proportioning valve. I bought a remanufactured MC. It doesn't have the residual values. The one thing I haven't been able to learn from all the brake posts is: Should the residual valve be placed in the line before or after the proportioning valve?

Thanks for any help with this.

Regards,
Gary
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Old 08-14-2007, 11:04 AM
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The residual valve goes right inside the MC at the line connection - on the side that feeds the drum brakes. Look inside the hole - you should see it in there. That makes your answer "before."
Often rebuilders take them out, then drum brakes won't work right.

If it has both, take out the disc side.
If it has none - take the MC back to where you got it and get one with the valve.
Cheapie junk parts stores like AutoChina usually don't use rebuilders that do it right - and they don't last long anyway.
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Old 08-14-2007, 11:12 AM
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An in-line valve should be placed after the proportioning valve. I got this info from a schematic from Pirate Jack's Brakes. I guess it really depends on the configuration the proportioning valve was designed for. The schematic I'm referencing is based on a modern booster/cylinder design, with a master-mounted proprotioning valve- similar to the 80's Chevy seup.


In a disc/drum setup, no residual valve is needed for the front discs, and a 2lb valve is recommended to give better feel to the rear drums.
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Last edited by Randyzzz; 08-14-2007 at 11:34 AM.
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Old 08-14-2007, 01:07 PM
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For years before disc were used, ALL master cylinders used a residual valve. Reason being is to keep slight hydraulic pressure against the flared lip cups, keeping the seals tight. That pressure kept the seals inflated, the springs overcame that pressure to retract the shoes.
When stopping, the shoes had to have enough pressure to overcome the springs and then move into place - about 10 lbs.

Along came discs - that same residing pressure held the pads against the discs, causing premature wear, dragging brakes etc.
Discs need to have the pressure go to 0. ANY pressure at all pushed the pads against the rotor, causing drag.
The disc's seals are square O rings, no need to keep them inflated, but the flared lips seals of drum brakes needs it.
So essentially the line pressure builds up until the drum brake's springs are overcome and the shoes begin to touch the drum. The disc's pads were already touching the rotors, no movement is needed there, so the hold-off or "proportioning" valve keeps pressure off the disc calipers. Then as the pressure is increased more, above the prop valve spring, BOTH the calipers and the cylinders get equal hydraulic pressure, hopefully stopping the vehicle properly.
The "balance" between braking forces between drums to discs is actually determined by the area of the caliper cylinders vs. the area of the wheel cylinders, and the friction materials - pad and shoe linings, as well as the friction material itself.

That's why mixing and matching brake systems from different vehicles sometimes works great, sometimes works terrible.


Master cylinders used to have the check valves in the line opening, usually a little rubber flap.
First thing ya know parts houses combined parts numbers to streamline their inventories - some MC's with the valves got installed on disc brakes - causing problems, lawsuits etc.
So - rebuilders to CYA started removing ALL check valves from ALL MC's.

Then drum brakes quit working like they should - but no lawsuits!
You've seen lots of posts about air getting in brake lines - now you know why. With no residual pressure lips seals collapse and let air in.
(Ever push on a drum brake vehicle after it sat a few weeks? First pump pedal to the floor, a pump or two later they work again. Not magic or a talented foot - the residual had bled down. Without the valve, that happens every time you step on the pedal.)

Quality parts houses still carry MC's properly fitted with the check valves - usually.

Yes, there are aftermarket in-line check valves you can buy - Jeggs etc - for custom installations when you are starting from scratch. But if you use the correct MC with the valves installed, there's no need.
Caution - If you use 2 check valves it's too much residual pressure - the shoes will drag.

Proportioning valves need far more pressure to operate than the 1-2 lbs a residual valve holds, so where it's placed is not important.

Unlike popular belief - The purpose for the proportional valve is to keep the discs from getting pressure until the shoes have a chance to move out against the drum. Disc pads don't have far to move.
If no prop valve is used, the discs start braking before the drums, making things squirrely.

The word "proportioning" valve is actually a bad word for it - a better word would be "hold-off" (Chrysler tried to use that word, everyone went nuts!) - a prop valve does not create or restrict pressure to one end or the other like the word implies. It simply "holds off" pressure to the front discs until the pressure is enough to overcome spring tension on the drum brake shoes.

Over the years an awful lot of information on the web has been based on false info.

Last edited by RRich; 08-14-2007 at 02:19 PM.
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Old 08-14-2007, 10:20 PM
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Perfect explanation, RRich. I was giving a very abbreviated answer since I was at work. (I also assumed that the master being used was not stock, since disc/drum is not a stock configuration.)
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Old 08-14-2007, 11:53 PM
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Yes, Rrich, thanks for the explanation. When I bought the jeepster the brakes worked great. In fact, when we took it for a ride before I bought it, Pete purposely got it up to 50 and locked the brakes. At least the front ones locked. And the brakes did work fine until I replaced a bent rear axle. They always were a bit soft after that. Hopefully I just need to bleed them through 10 - 20 times as you suggested in a previous post. If it is the MC, hopefully also that he has it set up right and I just need to replace worn out stuff and not reconfigure the whole system. Now that I have some clues I can check for the proportioning valve. I picked up an MC at Merle's that is made by Raybestos, or something like that, and it is new and not a reman. It looked like it had the Residual vales. We tried calling the manufacture but we were on hold too long. I will try them again tomorrow. If both are present then I will screw a screw into the front (disc) port and yank the brass funnel looking piece and remove the residual valve which should be under it. I understand it is no more than a little rubber diaphragm.

Just as a note, I don't know if it is fact or fiction, but I did read on the site of one of the valve manufactures that a 2lb residual valve only needs to be used on disk brakes that have the MC mounted lower than the disk brake calipers. This site said some custom builders mount the mc low so it can't be seen. Again, don't know if it is jive, but they said the test is to jack, it up, stomp the brake and release and spin the wheel. The brake should have some friction. Then, open the bleed screw, let out any pressure, close the screw and spin the wheel again. Should be no friction.

Regards,
Gary
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Old 08-15-2007, 05:57 AM
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Quote:
Originally Posted by RRich View Post
Over the years an awful lot of information on the web has been based on false info.
But if I read it on the internet it HAS to be true!!! (That was a great and accurate overview of hyrdraulics on your part. Thanks!!!)
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Old 08-15-2007, 09:22 AM
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He He - The old saying "paper does not reject ink" needs to be updated for the internet.

As far as the check valve needed on discs when the MC is lower than the caliper - Hmmm, on a Jeepster I think it would be better to turn the Jeepster right side up!

Obviously that's really low. I'd speculate the reason is gravity is trying to drain all the fluid back to the MC. That may create a slight vacuum at the caliper, then the square O ring probably lets air in.
Normally air bubbles can work themselves out by rising up the lines to the MC, but if the MC's lower it'll all collect in the caliper.

Good practice is to make sure all the lines point "up" to the MC without any bends or traps where bubbles can get trapped. That also includes hydraulic clutch systems. It makes the system sort of "self bleeding."
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