Rear brakes locking up after front disc conversion - Page 3 - Vintage Mustang Forums
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post #31 of 58 (permalink) Old 05-30-2019, 09:04 PM
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Or spend $10 more and get the best low cost flaring tool I've found. This one produces beefy flares as good as the high end tools:

https://www.eastwood.com/eastwood-on...16-tubing.html
That tool looks the business, but only for 3/16 line. Every time I use my double flaring tool I think it may be the last so I'm going to stick with my 1979-1980 vintage "Old Forge" double flaring tool. Sometimes aggravating to use but always getting the job done. A little Google reveled that they are still around and possibly still made in M'erica!
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post #32 of 58 (permalink) Old 06-03-2019, 08:14 PM Thread Starter
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post #33 of 58 (permalink) Old 06-03-2019, 11:28 PM
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I think the master cylinder you have listed MAY be a power brake unit (although it says "universal"). Question whether it has the retaining groove for the manual brake pushrod. I have always used the Raybestos MC36440 master for manual disc/drum apps.

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post #34 of 58 (permalink) Old 06-04-2019, 01:11 AM
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Having read this thread, I now have some questions!

1) What is the function/purpose of the 10# residual valve in the rear brake line?
2) Does the commonly-used '74 Mav disc/drum MC have a residual valve built in?

Just curious. I have the '74 Mav disc/drum MC plumbed with the fronts going to the stock junction block and the rears plumbed through a Wilwood proportioning valve and then straight to the brakes. Can't say I have any complaints but I'm always keeping an eyeball out for something that I missed.


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post #35 of 58 (permalink) Old 06-04-2019, 10:48 AM
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Originally Posted by Kelly_H View Post
Having read this thread, I now have some questions!

1) What is the function/purpose of the 10# residual valve in the rear brake line?
2) Does the commonly-used '74 Mav disc/drum MC have a residual valve built in?

Just curious. I have the '74 Mav disc/drum MC plumbed with the fronts going to the stock junction block and the rears plumbed through a Wilwood proportioning valve and then straight to the brakes. Can't say I have any complaints but I'm always keeping an eyeball out for something that I missed.
1) Good question. The combo valve shown should have an integral RPV.
2) The much recommended 74 Maverick master cylinder does not have an integral RPV. CSRP reproduces this master cylinder with a removable RPV in the rear brake circuit.
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post #36 of 58 (permalink) Old 06-04-2019, 11:03 AM
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Originally Posted by Kelly_H View Post
Having read this thread, I now have some questions!

1) What is the function/purpose of the 10# residual valve in the rear brake line?
2) Does the commonly-used '74 Mav disc/drum MC have a residual valve built in?

Just curious. I have the '74 Mav disc/drum MC plumbed with the fronts going to the stock junction block and the rears plumbed through a Wilwood proportioning valve and then straight to the brakes. Can't say I have any complaints but I'm always keeping an eyeball out for something that I missed.
I had the exact setup you have for 15 years and the brakes worked flawlessly. When I did the Explorer rear disc swap a couple of years ago I learned about the residual valve that I never had. Of course, I donít need in now with 4 wheel disc, but it is strange to hear that I was apparently driving a death trap for over a decade, lol (sarcasm)

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post #37 of 58 (permalink) Old 06-04-2019, 12:40 PM
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Originally Posted by Kelly_H View Post
1) What is the function/purpose of the 10# residual valve in the rear brake line?
2) Does the commonly-used '74 Mav disc/drum MC have a residual valve built in?

Just curious. I have the '74 Mav disc/drum MC plumbed with the fronts going to the stock junction block and the rears plumbed through a Wilwood proportioning valve and then straight to the brakes. Can't say I have any complaints but I'm always keeping an eyeball out for something that I missed.
The #10lbs keeps the cylinders charged so the shoes stay closer to the drums for the next stop. Without it the springs will push the pistons into the cylinder and thus have to travel out more before they touch during the next stop.
Since the fronts do most of the braking you may never notice the rears aren't helping much if at all during a normal stop. During a panic stop they might work but thats just because you are pressing so hard overall.
Tune for flatness or just the slightest nose dive while braking and after several NORMAL or slow stops check that the rears have heat in them.

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post #38 of 58 (permalink) Old 06-04-2019, 01:47 PM
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The #10lbs keeps the cylinders charged so the shoes stay closer to the drums for the next stop. Without it the springs will push the pistons into the cylinder and thus have to travel out more before they touch during the next stop.
Nope. Complete BS and yet another nonsensical thing that circulates throughout the web.
A 10# valve is NOT going to overcome drum brake spring pressure and keep shoes closer to the drum friction surface.
No how, no way.

The residual valve merely keeps pressure in the line so that with the next brake pedal application, you don't have to "recharge"
that brake line with fluid.... that may have drained back into the master cylinder.
This is important on most drum applications and really important when the master cylinder is below the level of the drums,
like on some classic hot rod applications.

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post #39 of 58 (permalink) Old 06-04-2019, 06:58 PM Thread Starter
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I think the master cylinder you have listed MAY be a power brake unit (although it says "universal"). Question whether it has the retaining groove for the manual brake pushrod. I have always used the Raybestos MC36440 master for manual disc/drum apps.
According to what I saw in the Q and a section of the parts there were quite a few people that said they would work and has work on a 66 Mustang manual brake disks/drum set up. Hopefully this all works so I don't have to dump more money into just being able to have the rear brakes work properly
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post #40 of 58 (permalink) Old 06-04-2019, 07:23 PM
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Nope. Complete BS and yet another nonsensical thing that circulates throughout the web.
A 10# valve is NOT going to overcome drum brake spring pressure and keep shoes closer to the drum friction surface.
No how, no way.

The residual valve merely keeps pressure in the line so that with the next brake pedal application, you don't have to "recharge"
that brake line with fluid.... that may have drained back into the master cylinder.
This is important on most drum applications and really important when the master cylinder is below the level of the drums,
like on some classic hot rod applications.

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YES!
The residuals will keep a line charged with 10 pounds on the drum circuit-NOT enough force to overcome the spring pressure. The 10 pounds does keep you from pushing that fluid back to the cylinder AGAIN to make the cylinders activate.
Shoe distance to drum is achieved by correct adjustment. 85% of folks have no idea of how to adjust shoe properly.
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post #41 of 58 (permalink) Old 06-04-2019, 08:00 PM
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Well it sounded good to me. maybe i have weak springs 10lbs is worth something though.
The situation i described about the shoes not engaging as early as it should without a RPV i have seen to be true though.

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post #42 of 58 (permalink) Old 06-04-2019, 09:17 PM
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YES!
The residuals will keep a line charged with 10 pounds on the drum circuit-NOT enough force to overcome the spring pressure. The 10 pounds does keep you from pushing that fluid back to the cylinder AGAIN to make the cylinders activate.
Shoe distance to drum is achieved by correct adjustment. 85% of folks have no idea of how to adjust shoe properly.
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Nope. Complete BS and yet another nonsensical thing that circulates throughout the web.
A 10# valve is NOT going to overcome drum brake spring pressure and keep shoes closer to the drum friction surface.
No how, no way.

The residual valve merely keeps pressure in the line so that with the next brake pedal application, you don't have to "recharge"
that brake line with fluid.... that may have drained back into the master cylinder.
This is important on most drum applications and really important when the master cylinder is below the level of the drums,
like on some classic hot rod applications.

ex-Global West GM
1991-1995
So second question then - why is this good? Since we spend so much time dialing in proportioning valves to specifically ensure that the rear drums DON'T come on too hard, too fast - seems like keeping residual pressure in the line to allow the rears to activate faster would be a bad thing?


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post #43 of 58 (permalink) Old 06-04-2019, 11:43 PM
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So second question then - why is this good? Since we spend so much time dialing in proportioning valves to specifically ensure that the rear drums DON'T come on too hard, too fast - seems like keeping residual pressure in the line to allow the rears to activate faster would be a bad thing?
Kelly- Speed of actuation and % of actuation are two distinctly different qualities.


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post #44 of 58 (permalink) Old 06-05-2019, 07:13 AM
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So second question then - why is this good? Since we spend so much time dialing in proportioning valves to specifically ensure that the rear drums DON'T come on too hard, too fast - seems like keeping residual pressure in the line to allow the rears to activate faster would be a bad thing?

When I added a residual valve, I noticed slightly better pedal height where the brakes engaged. I also found I got less nose dive, likely because the rears were engaging earlier. TO sum it up, it was worth the $10, I plumbed it in right at the master just past the first bend.


I also plumbed in a later model proportioning valve at the same time which limits the magnitude of the pulse to the rears for a microsecond while the weight transitions to the front keeping the rears from locking up. By the time the car settles weight on the rear again, you have full brake power at the rear. With your adjustable portioning valve, I would imagine you could back off and bias slightly more to the rear and get better stopping all around. I can brake noticeably later in a corner on an autocross due to these changes. Its controllable and nicely balanced.

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post #45 of 58 (permalink) Old 06-05-2019, 09:55 AM
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Kelly- Speed of actuation and % of actuation are two distinctly different qualities.


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Hmm, I guess I am not really clear on how a prop valve works then. To me, it creates a restriction in the flow but does not ultimately impact the volume of fluid that ends up in the line. Essentially, the restriction creates a reduction in rate of flow, no? It should not affect total end % of actuation, rather just biasing less % of actuation for X amount of braking because of the delay in flow/pressure buildup due to the restriction?

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When I added a residual valve, I noticed slightly better pedal height where the brakes engaged. I also found I got less nose dive, likely because the rears were engaging earlier. TO sum it up, it was worth the $10, I plumbed it in right at the master just past the first bend.


I also plumbed in a later model proportioning valve at the same time which limits the magnitude of the pulse to the rears for a microsecond while the weight transitions to the front keeping the rears from locking up. By the time the car settles weight on the rear again, you have full brake power at the rear. With your adjustable portioning valve, I would imagine you could back off and bias slightly more to the rear and get better stopping all around. I can brake noticeably later in a corner on an autocross due to these changes. Its controllable and nicely balanced.
Neat, that makes sense to me. Basically your starting pressure baseline is just slightly higher while the rate of flow to the rears beyond that during braking is controlled by the prop valve?

It is crazy that a 10# valve can make a difference given how much pressure ultimately ends up in those lines!
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Wrecked and rebuilt even better
289 v8, 4-speed, 3.25 9" rear, goodies and stuff.

See my travel blog here for my adventures: http://mapandamustang.blogspot.com/
Over 50,000 miles of North American roads driven on road trips since 2014. More always in the works - stay tuned!

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