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Low brake pressure

Mackman

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I have a 69 GTX, A33 car which when I purchased it had 11" drums at all four wheels. The build sheet reads B31 brakes, which I think would be hevi duty manual drums, but someone has converted it to power. Braking action was poor to the point of being scary, but no "Brakes" light on the dash ever lit.
I decided to rebuild the engine this winter, so as long as I had the K member and engine down, I converted it to Dr. Diff 11 3/4" Stage II front disc kit. I installed his disc/drum proportioning valve, rebuilt the rear wheel cylinders, installed a new 15/16" master, and converted to D.O.T. 5 fluid.
Started the rebuild, ran it for break-in, and everything runs good. Had a good pedal feel, not too hard, but the "Brakes" light on the dash would come on at the bottom of the pedal stroke, which is not at the floor. The booster is working, vacuum is good, cam is a Comp Cams CRB XE 262H-10.
I thought maybe brake volume wasn't enough, so I installed a 1 1/32" master. "Brakes" light still comes on near the bottom of the pedal stroke, everything bled good, pedal is not mushy and doesn't pump up. I bench bled both masters.
Today I checked brake pressure- 600 psi at all four wheels. I think 600 psi would be fine for the rear drums, but low for the single-piston calipers in front. I have the drums adjusted well out. Salt is still on the road here in NY so I can't take it out.
I would like some opinions, please. What may be causing this low pressure?
 
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I am using the booster that was on the car. I believe the linkage inside the car is correct for power brakes. The bleeders in the front are at the highest point. Pressure reads the same at all four wheels.
 
Disc brakes need more pressure than drums. 600 PSI is way too low. You need to be well over 1000 PSI to have adequate stopping force. The "Brake" light is dual function. It tells you when the parking brake is on and when there is a pressure difference in the system.
There is a shuttle in the proportioning valve that moves back and forth between the front system and rear.

22 6B.JPG
22 6C.JPG

This is a drum-drum block but the principle is the same, Maybe the valve is defective and instead of allowing maximum pressure to the front, it is blocking it?
 
From what I read. You should install a metering block and an adjustable proportioning valve to change the brake bias. Use the adjustable proportioning valve on the rear brake line. The metering valve is so that you can maintain higher pressure to the front calipers and lower pressure to the rear line if using drum brakes. They make and sell a metering valve that is specific for front caliper and rear drum applications.

This is the part.
And this is what it looks like.
s-l500.jpg
 
I have a 69 GTX, A33 car which when I purchased it had 11" drums at all four wheels. The build sheet reads B31 brakes, which I think would be hevi duty manual drums, but someone has converted it to power. Braking action was poor to the point of being scary, but no "Brakes" light on the dash ever lit.
I decided to rebuild the engine this winter, so as long as I had the K member and engine down, I converted it to Dr. Diff 11 3/4" Stage II front disc kit. I installed his disc/drum proportioning valve, rebuilt the rear wheel cylinders, installed a new 15/16" master, and converted to D.O.T. 5 fluid.
Started the rebuild, ran it for break-in, and everything runs good. Had a good pedal feel, not too hard, but the "Brakes" light on the dash would come on at the bottom of the pedal stroke, which is not at the floor. The booster is working, vacuum is good, cam is a Comp Cams CRB XE 262H-10.
I thought maybe brake volume wasn't enough, so I installed a 1 1/32" master. "Brakes" light still comes on near the bottom of the pedal stroke, everything bled good, pedal is not mushy and doesn't pump up. I bench bled both masters.
Today I checked brake pressure- 600 psi at all four wheels. I think 600 psi would be fine for the rear drums, but low for the single-piston calipers in front. I have the drums adjusted well out. Salt is still on the road here in NY so I can't take it out.
I would like some opinions, please. What may be causing this low pressure?
You need to upgrade to a dual diaphragm booster. Your front discs need about 900 PSI. The stock drum booster won't provide the needed pressure.
 
You need to upgrade to a dual diaphragm booster. Your front discs need about 900 PSI. The stock drum booster won't provide the needed pressure.
Not completely true. Yes the booster can aid and allow more pressure with less leg effort. However it does not dictate the maximum pressure. All you have to do is apply more peddle pressure and it will increase. A booster is just an assist.
 
Not completely true. Yes the booster can aid and allow more pressure with less leg effort. However it does not dictate the maximum pressure. All you have to do is apply more peddle pressure and it will increase. A booster is just an assist.
Been there, done that! I tried using the drum booster that came with my 68 Charger. You are correct when you say a booster is just an assist. However it is a proven fact that discs need more assist than a drum booster can provide. I could use both feet on my brake pedal and could not lock the front discs with the drum booster which was rebuilt at the time. I'm not the first or the last to make this mistake. Tons of disc brake swappers have tried it and there are tons of wrong info out there. I swapped in the dual diaphragm booster and all of my brake issues went away. Mopar fixed this problem for us many years ago but we still seem to want to defy the odds.
I googled the question of how much pressure is required for disc brakes as opposed to drum brakes. The answer was drum brakes require about 400 lbs while disc brakes need closer to 900 lbs.
There is a formula for brake boosters and it uses a single 8" booster for reference. The math works like this......4" X 4" X 3.14=50.24 square inches X 8=401.9 pounds of force. A dual diaphragm booster doubles it to 803.8 pounds. It also said 4 wheel discs require a dual diaphragm booster.
That said, a dual diaphragm 9" booster provides 1144.5 lbs. (even a 9" single diaphragm only gives you 572 lbs of pressure. May be enough for some small discs but not the larger ones)
 
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A dual diaphragm booster doubles it to 803.8 pounds. It also said 4 wheel discs require a dual diaphragm booster.
That said, a dual diaphragm 9" booster provides 1144.5 lbs.
I don't understand your math here.
I've dealt with braking that feels inadequate despite having big rotors and calipers. I have a 1975 Dart brake booster, a single diaphragm unit.
Maybe all along, my issue is too small of a booster?
 
I don't understand your math here.
I've dealt with braking that feels inadequate despite having big rotors and calipers. I have a 1975 Dart brake booster, a single diaphragm unit.
Maybe all along, my issue is too small of a booster?
Here is the formula for a single diaphragm 9" booster (4.5 X 4.5 X 3.14=63.585 X 9= 572.2 lbs of force.)
If you have a dual diaphragm 9" booster, you double the force to 1144.5 lbs
I mentioned this formula in your thread as well back a while. Your 75 Dart booster is not up to the task. It is giving you around 400 lbs of boost and you need double that at least. Especially with discs in the rear as well. I agree with pnora when he says "all you have to do is apply more pressure" when you have manual brakes but for some reason, when you put a vacuum powered booster in line, all you get is what that booster will give you.
 
Disc brakes need more pressure than drums. 600 PSI is way too low. You need to be well over 1000 PSI to have adequate stopping force. The "Brake" light is dual function. It tells you when the parking brake is on and when there is a pressure difference in the system.
There is a shuttle in the proportioning valve that moves back and forth between the front system and rear.

View attachment 1438797View attachment 1438798
This is a drum-drum block but the principle is the same, Maybe the valve is defective and instead of allowing maximum pressure to the front, it is blocking it?
Hey, Kern. I'm pretty sure the brake light is coming on because of low pressure to the calipers. I think maybe this proportioning valve from Cass is not doing the job. I didn't think I would have to go to an adjustable valve to get this set-up to work. And you are right in that those calipers need to see at least 1200 psi to function correctly.
 
Maybe I should put a nut on that upper stud, huh?
 
This is the booster I have, and the backing plate and linkage appear to be correct.

View attachment 1438939

View attachment 1438940
That is the single diaphragm booster for drum brakes. You need one like this. It is a direct replacement although it is aftermarket and has a larger rubber boot on the back. You will need to enlarge the hole in your firewall a little to install it. It comes with the backing plate and linkage but is the same as what you already have.
1966-70 Mopar OE Bendix Style 8" Dual Power Brake Booster for Mopar B Bodies | eBay
 
Hey, Kern. I'm pretty sure the brake light is coming on because of low pressure to the calipers. I think maybe this proportioning valve from Cass is not doing the job. I didn't think I would have to go to an adjustable valve to get this set-up to work. And you are right in that those calipers need to see at least 1200 psi to function correctly.
Brake light is on from either 1-To much travel in the calipers or shoes or 2- An imbalance on the hydraulic system.
 
I understand that having air in the system results in greater pedal travel. The proportioning valve will trip the warning light because of that.
These units are great for bleeding brakes:

22 4 VC.JPG
22 4 VF.JPG

Brakes with TOO much pedal travel is a pisser to drive but a larger bore master cylinder with really short travel can be TOO hard to push for some people.
 
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