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Discussion Starter #1
On a dual master cylinder (the type used on front disk, rear drum Fords in the late 1960s and 1970s), if I plug the port on the master cylinder going to the rear, the front system would still operate, correct? The reason I ask is I'm trying to diagnose a problem, and I want to remove the rear system from the equation (no, I'm not driving the car like this).
 

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Yes, but...

You will loose the ability to fully apply the front brakes because the rear plunger does not have a relief, and when the small amount of fluid fully compreses the fronts may just have a lot further they can go...

I can see it in my head but not put it into words. The secondary portion of the MC piston will compress the fluid to the point that you have it capped adn the fluid will compress no more therefore you will not push any more fluid to the front brakes...

See what I mean?

How about this. Remove the rear brake line then route the MC exit back into the MC like you were bleeding it then you will fully utilise your front portion of the brakes...

CAN ANYONE please explain what I am thinking... It's been a long hot day...


:D

FE
 

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inside the MC there are 2 springs. One that returns the pedal, it is located at the very front of the MC. The other is located between the rear piston and the front piston. So, if you capped the front port of the MC, the fluid would be pressurized but eventually pushing the pedal harder will compress the spring that's in between the rear piston and front piston. But if you cap the rear port, it won't move. Keep in mind that you cannot compress liquids unless it has a gas bubble in it (e.g. air). You can pressurize the liquids but they won't compress (which is why rods bend when water is on top of pistons....)

If I'm wrong, someone correct me.
 

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inside the MC there are 2 springs. One that returns the pedal, it is located at the very front of the MC. The other is located between the rear piston and the front piston. So, if you capped the front port of the MC, the fluid would be pressurized but eventually pushing the pedal harder will compress the spring that's in between the rear piston and front piston. But if you cap the rear port, it won't move. Keep in mind that you cannot compress liquids unless it has a gas bubble in it (e.g. air). You can pressurize the liquids but they won't compress (which is why rods bend when water is on top of pistons....)

If I'm wrong, someone correct me.
Nope you are right.
 

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If you are trying to eliminate a certain part of the brake system as the cause of a low pedal, I've had good luck with simply clamping the rubber brake hoses. The key is to do it GENTLY. There are tools for this but needle-nose vise grips will work. Slip a piece of fuel line over the brake hose in the area to be clamped for added protection.

Clamp each hose (rear, l-front,r-front) one at a time. If the pedal improves then there is a problem downstream of your clamp.
 

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Discussion Starter #8
Ok - I'll use the bench bleeder nipple and run the line back into the reservoir. The problem is a soft pedal (well, no pedal at all). I assume there is a leak, but since I posted, I've replaced ALL the lines, T's, prop valve, and wheel cylinders. I've bled this thing through 2 quarts of brake fluid and there's no air in there. The master cylinder was new too, but now I'm pretty sure it must be a "bad" rebuild because I should have some pedal at least. I want to try just the front and then the rears to see if there is any difference. Frustrating, but I'm concluding it's a bad "new" master cylinder.
 
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