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I have 4 wheel disks on my 68 Firebird but can't lock up the brakes. It stops ok, better when have some speed or first hit the brakes. But best case the passenger rear will lock up a little. New master cylinder and vacuum booster, stock automatic proportioning valve. 383 doesn't make much vacuum though. How do I determine if I have sufficient vacuum or maybe need a manual proportioning valve?
 

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A inline break pressure gauge will let you find issues. Bit of fluid, a helper, and a afternoon. You can apply vacume to the booster with a mighty vac with the car off to find leaks in the diaphragm
 

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There is no ‘automatic’ or ‘manual’ prop valve.
Disc/drum or disc/disc use a prop valve. Drum/drum use a distribution valve.
Sounds like you don’t have enough vacuum. Usually you need at least 12-14 hg.
Post a pic of your prop valve.
 

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**383 doesn't make much vacuum though. How do I determine if I have sufficient vacuum**

? is the peddle hard at anytime useing the brakes ?

You can check he vacuum from the carb port, vacuum is the same no matter where it is measured 18in at least.

Could remove the line from the booster and check it, however ID is larger than a vacuum gauge line.

Low vacuum & large bumpy cam is normal. Example, my motor pulls a grand total of 7". To solve an aux vacuum pump will need to be installed.

Pep
 

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Discussion Starter #6
Thanks for all the replies. I'm confident the lines are bled of air. Pedal isn't hard really. Checked intake vacuum and realized, duh, I can read vacuum on my Proflo digitally. It's old but that's one thing it does. I did some speed and hard braking while watching the vacuum. Vacuum is 17-20" when I have some speed and let off the throttle. But only 7-10" at lower rpm and idle, and 2-5" at WOT. So brakes are good under some conditions and poor at others. That's something the vacuum pump would fix. I think maybe the front pads needed to bed also because braking improved by the time I was done test driving. Managed to lock most wheels a couple times.:D

RWENUTS can you demystify my prop valve? Is it just hard set to a proportion? What is that proportion? What is the wire for?
https://www.summitracing.com/parts/sum-760185?seid=srese1&cm_mmc=pla-google-_-shopping-_-srese1-_-summit-racing&gclid=EAIaIQobChMIua_smuOo4QIVkLrACh1zDgBPEAQYAyABEgJO1vD_BwE
 

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IIRC they’re usually set for 80% front, 20% rear. On a disc/drum set up. Disc/disc might be the same. I’m not sure what the percentage is however the percentage will be higher at the front.
The wire is for your brake warning light. If you loose fluid from a leak the piston in the prop moves and trips the switch.

Even though you might need a pump you could try a vacuum tank first.
 

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Discussion Starter #8
IIRC they’re usually set for 80% front, 20% rear. On a disc/drum set up. Disc/disc might be the same. I’m not sure what the percentage is however the percentage will be higher at the front.
The wire is for your brake warning light. If you loose fluid from a leak the piston in the prop moves and trips the switch.

Even though you might need a pump you could try a vacuum tank first.



Thank you! So given that bias, shouldn't I expect the front to lock up and the rear not so much?

I agree on the tank too, I like simple.
 

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Yup!
More braking on the front due to weight transfer.

Thanks. So the stock disk/disk prop valve is new, assumed working. What could be causing the rear to lock up first? I suppose a lot of things like weight transfer, tires, weight balance? I think I need the adjustable prop valve. I've seen them installed on the rear brake line, after the stock disk/disk prop valve. Is that right? I guess it would make the rear 20% adjustable to 0%.
 

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"I'm confident the lines are bled of air. Pedal isn't hard really."


Well hate to tell you but you have to get this part right first. Until you have a hard pedal then you have air in the system. I asume you bench bled the master before starting the bleeding procedure. Power brakes are simply an assist so that the amount of leg effort to brake the car is reduced, it has nothing to do with whether the hydraulic part of the braking system is functioning correctly.



Less than a hard pedal = air in the lines.
 
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Thanks for all the replies. I'm confident the lines are bled of air. Pedal isn't hard really. Checked intake vacuum and realized, duh, I can read vacuum on my Proflo digitally. It's old but that's one thing it does. I did some speed and hard braking while watching the vacuum. Vacuum is 17-20" when I have some speed and let off the throttle. But only 7-10" at lower rpm and idle, and 2-5" at WOT. So brakes are good under some conditions and poor at others. That's something the vacuum pump would fix. I think maybe the front pads needed to bed also because braking improved by the time I was done test driving. Managed to lock most wheels a couple times.:D

RWENUTS can you demystify my prop valve? Is it just hard set to a proportion? What is that proportion? What is the wire for?
https://www.summitracing.com/parts/sum-760185?seid=srese1&cm_mmc=pla-google-_-shopping-_-srese1-_-summit-racing&gclid=EAIaIQobChMIua_smuOo4QIVkLrACh1zDgBPEAQYAyABEgJO1vD_BwE
"So brakes are good under some conditions and poor at others."
You don't have enough vacuum to consistantly operate the booster. Possibly cured with a reservior cannister or you may have to go with a vacuum pump.
I would also double check the brake bleed.
 

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I did some speed and hard braking while watching the vacuum. Vacuum is 17-20" when I have some speed and let off the throttle. But only 7-10" at lower rpm and idle, and 2-5" at WOT. So brakes are good under some conditions and poor at others.
This is a classic problem of a hefty cam, you are not developing enough vacuum to supply the booster period. The reason the brakes work, sort of, sometimes, now and then while out driving. Is because when you are decelerating the motor will develop some good vacuum, but only off throttle, as seen.

So using the brakes as in traffic slow rolling, or multi applications in the short period of time. The low vacuum becomes highlighted.... extremely noticeable

To test just go out, put it in gear roll down the drive way, try to apply the brake 5 times in a row. Bet you do not get 3 applications, before the peddle gets HARD.. no boost, low of vacuum.

Good luck, it is an easy problem to solve, will take few bucks depending on the route you take.

Ruff figures 300 - 500, build your own 200


This is a myth that comes up all the time''reservoir canister" These do not help the breaking, it only affects the recovery time, of the system.

What is being said ............. if the system has 7 inches of vacuum and towing a 60 gal compressor tank behind the car with 7inches of vacuum pulled.

. Your frigging brakes will still be hard. It will take an hour for the system to pull another 7 inches of vacuum to recover and still be useless

Now if the only reservoir canister, in the system and that being the booster, you will pull 7 inches is seconds and still be useless ............ solution vacuum pump . period
Pep
 

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Just solved my identical problem today (see "Mustang II brakes thread). A tank alone helped, and in your case might work well since you sometimes have higher vacuum. But in my case, a pump did the trick. Pedal is no longer hard and I can lock up the brakes.
 

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The combination of, soft pedal, vacuum booster, weak vacuum, and poor braking don't add up.

With a booster and weak vacuum I would expect a very hard pedal with poor braking.
I also would try bleeding again. brake fluid is cheap.
I backed up and read that you now can sometimes lock them up, but is the pedal still soft? With it running can you push the pedal to the floor pushing really hard? Hard like you are heading for a brick wall!

What size bore is the master cylinder?
 

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Addressing the magic can .....

Scenario: Coming off the highway now entering stop and go traffic, or car show entry traffic:

First off, we can all agree the booster and the magic can, both have check valves and will hold whatever vacuum is available, equally.

Deceleration, vacuum pulls down to 25 inches, both are now storing that figure.

Motor pulls 7-10 inches of vacuum. First hit on the brakes it takes 10 inches of the stored resources, an example only.

Math and real life says both booster and the magic can now have 15 inches of vacuum.

Still jammed in traffic, the second brake application will bring the booster and the magic can down to 5 inches.

The motor as the only the source of vacuum, total available vacuum will now be max of 7 - 10 inches, when replenished.

The myth I speak of, the magic can is not a 100% low motor vacuum fix, the very best it is a band-aid. That will produce a hard pedal when least expected, and maybe worse.

Second the nonsense of having a magic can and auxiliary vacuum source as a back up. Is just nonsense. The only thing the magic can will do is extend the recovery cycle time of the pump, and will actually cause undo wear of the unit.

Pep
 

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I have not read all previous posts, but my take is you are most likely short on vacuum as 4 wheel discs tend to have this problem and there is also a possibility that your brake booster is too small too boot...

That is why so many folks are turning to vacuum back up systems for help.
 

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Thanks for all your replies. I ran the Spokane County Raceway road course and got a better feel for the brakes. They're awesome! Best brakes ever for the money and my own custom adaptation. Vacuum is 18+ during track driving and they felt well balanced. But my vacuum is poor in town and traffic. Everybody who said it was low vacuum, that's what I conclude now. And pads needed bedding. So I'll need a vac pump or hyd brake booster if I want any improvement now.



Here's the brakes...


Here's the track day...
 
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