Brake Issues

[Moparfiend]

Sorry for the long text here but wanted to explain the issue as best I could….

I am having some brake issues, namely I feel that I’m not getting any power assist. I believe it’s potentially a mismatch of master cylinder, bore size and rear cylinder bore size as I believe the brakes were working before the rear wheel cylinders were replaced. I got them from Rick Ehrenberg and I believe they ate not the stock rear wheel cylinder diameter. This was supposed to prevent rear wheel lock up. Thats what he mentioned to me anyway.

Also in the mix of variables is I rebuilt my rear brakes ( fronts are disc btw). I am not 100% sure the rear is fully adjusted and may still need to be tightened more?. I am posting a picture of the pads with only several miles of test driving. Can anyone tell if the brake pads are starting to seat ok by the wear pattern? It’s a bit choppy but again its hardly had a chance to seat. I never felt any chatter was smooth just no power assist to stop.

Lastly I have re-bench bled the master cylinder and the rest of the hydraulics going from the rear passenger first to driver front last. Finally using gravity feed after first using my vac. I think there is no air in the lines but do have a little more peddle depression than I care for. So I triple check again tomorrow if possible.

Anyway I need to determine if the system bore ratios are incorrect. My master cylinder is aftermarket. So I don’t know the bore size but could measure. Again, I believe before I rebuilt the rear brakes and put on the new rear wheel cylinders the brakes seemed to have ok power assist if memory serves me.

Any inputs are very much appreciated in trying to detangle this issue.
Ron

 

[Sixpactogo]

I have more questions than answers but you mentioned you believe you are not getting enough power assist. I am wondering about your booster. Is it an aftermarket piece? Is it a dual diaphragm unit? Often times people miss-match parts that won't work together such as installing a single diaphragm booster after upgrading to front disc's and or not upgrading the brake distribution block to a proportioning valve. Disc brakes require about twice the PSI as drum brakes do. That is why the dual diaphragm booster was created along with the proportioning valve to regulate the pressure to the rear brakes. Without the correct combination of parts, you will never be happy with the stopping power.
Did your car originally have front discs?

 

[Moparfiend]

I have more questions than answers but you mentioned you believe you are not getting enough power assist. I am wondering about your booster. Is it an aftermarket piece? Is it a dual diaphragm unit? Often times people miss-match parts that won't work together such as installing a single diaphragm booster after upgrading to front disc's and or not upgrading the brake distribution block to a proportioning valve. Disc brakes require about twice the PSI as drum brakes do. That is why the dual diaphragm booster was created along with the proportioning valve to regulate the pressure to the rear brakes. Without the correct combination of parts, you will never be happy with the stopping power.
Did your car originally have front discs?

Yes the car was originally a front disc set up with I believe the single large piston Kelsey Hayes. Sorry I know that was a long intro to have to get through.

I did mention that this set was working ok before I rebuilt the rear brakes with the new wheel cylinders. Leading me to believe it might be the new wheel cylinders. Added to that that those wheel cylinders have a non standard bore.

 

[Moparfiend]

Is there an easy way to determine if I have a single vs duel diaphragm MC?
I found this comparison on another forum non-Mopar. Mine looks more like the duel on the left as opposed to the single pancake on the right. But I ain’t no expert…..

 

[Fran Blacker]

Do you have/know old rear wheel cyl size compared to new RWC size? Do you have self adjusting brakes in rear?

 

[Sixpactogo]

Is there an easy way to determine if I have a single vs duel diaphragm MC?
I found this comparison on another forum non-Mopar. Mine looks more like the duel on the left as opposed to the single pancake on the right. But I ain’t no expert…..
View attachment 1728016

I don't recognize the booster you have in your car but I do know about miss-matched parts not being user friendly. When I built my Niece a 70 Challenger, I upgraded to power disc front brakes from manual 4 wheel drums. I bought a booster that looked similar to the one you have. It was a 7" unit that was not up to the task and couldn't slide the wheels. I did some research and found I needed the dual 8" Bendix style booster. Once I swapped in the correct booster, her car stopped as it should. Here are a couple pictures. The first is the style I tried to use. The second one is the dual 8" Bendix style. I am not positive that is your problem but it was mine. Maybe call Ehrenberg and see what he has to say. He is pretty knowledable and has never led me astray. Good luck.

 

[WileE]

You say " I think there is no air in the lines "... well, that's got to be confirmed before anything else.

 

[Moparfiend]

You say " I think there is no air in the lines "... well, that's got to be confirmed before anything else.

I re-bled the brakes again today no air is now confirmed and I re-adjusted the rear brakes. Pedal pressure is good.
But still not getting much if any assist especially from 10mph to stop I have to stand on it.

 

[Moparfiend]

I don't recognize the booster you have in your car but I do know about miss-matched parts not being user friendly. When I built my Niece a 70 Challenger, I upgraded to power disc front brakes from manual 4 wheel drums. I bought a booster that looked similar to the one you have. It was a 7" unit that was not up to the task and couldn't slide the wheels. I did some research and found I needed the dual 8" Bendix style booster. Once I swapped in the correct booster, her car stopped as it should. Here are a couple pictures. The first is the style I tried to use. The second one is the dual 8" Bendix style. I am not positive that is your problem but it was mine. Maybe call Ehrenberg and see what he has to say. He is pretty knowledable and has never led me astray. Good luck.

View attachment 1728099

View attachment 1728100

Thanks yes I believe this is my problem. I have written to Erhenberg but no response yet. I hate just throwing parts at this but I am not sure there is anyway other way to determine compatibility…..

 

[Moparfiend]

Do you have/know old rear wheel cyl size compared to new RWC size? Do you have self adjusting brakes in rear?

I don’t know the wheel cylinder size of either the master cylinder or rear wheel cylinder. If I did I still wouldn’t know what is compatible or the correct ratio’s. Or if I have a single or duel booster….

 

[Some Car Guy]

I believe switching to a 7/8 wheel cylinder from like a 15/16 was a trick to balance out a system years ago. Been a looonng time so take that for what it’s worth.

Why did you change these? Maybe order a set of stockers off rockauto and go back to the size you had before that worked.

 

[Fran Blacker]

I don’t know the wheel cylinder size of either the master cylinder or rear wheel cylinder. If I did I still wouldn’t know what is compatible or the correct ratio’s. Or if I have a single or duel booster….

You said brakes worked fine before you changed WC's,correct? Some have think it's a spring, not sure. Can you look up wheel cylinder size, I can maybe help year, make, model and factory disc's?

 

[Moparfiend]

I believe switching to a 7/8 wheel cylinder from like a 15/16 was a trick to balance out a system years ago. Been a looonng time so take that for what it’s worth.

Why did you change these? Maybe order a set of stockers off rockauto and go back to the size you had before that worked.

Yeah that’s probably my next step easy and cheap to try. I changed over because I was rebuilding my rear brakes and wanted a new set as I had no idea how old the ones that were there were. After communicating with Rick he said get “these” so I did but the wild card is the booster. I don’t know the bore size. I will try and measure that. I hear I can do this by removing the MC away and getting a caliper in there?

 

[Don Frelier]

If the pedal is at the top and extremely hard to push then I doubt changing the rear wheel cylinders has much to do with lack of stopping power unless you were doing a bunch of braking in the rear previously and didn't know it?
Most of the stopping force is done in front.
I think the booster is questionable.

 

[Sixpactogo]

If, in fact, you have a single diaphragm booster, it is your problem. Even if it is a 7" dual diaphragm booster, it is not be supplying enough pressure to operate your brakes. You only get what that booster can give you no matter how much pressure you put on the pedal. Here is a formula that the manufacturers use. Drum brakes require about 400 psi while Disc brakes require closer to 900 psi. Lets say you have a single 8" diaphragm booster, here is the formula they use. 4" X 4" X 3.14 = 50.24 square inches X 8 = 401.9 pounds of force. (good enough for drum brakes) A dual 8" diaphragm booster doubles the 401.9 making 803.8 pounds of force.
A dual 7" booster uses this formula: 3.5 X 3.5 X 3.14 = 38.4 X 7 = 269.2 X 2 = 538.4 pounds of force. (good enough for drum brakes)
A single 9" booster uses this formula: 4.5 X 4.5 X 3.14 = 63.58 X 9 = 572.2 Pounds of force. (Good enough for drum brakes)
Using the formula for figuring out how much boost you get compared with the boost you need makes it much easier to choose what size booster you need. That is why when Chrysler tooled up for the front disc/rear drum, combination, they opted for the dual 8" booster and a proportioning valve in our B body cars. I believe in order to fix your problem, you need to upgrade your booster to an 8" dual diaphragm like the black unit I showed in post #6
Chrysler fixed a lot of these kind of problems before we even knew they were a problem. We only found out when we started miss-matching part that don't work together. Quiz Ehrenberg on this if you don't believe me.

 

[Moparfiend]

If, in fact, you have a single diaphragm booster, it is your problem. Even if it is a 7" dual diaphragm booster, it is not be supplying enough pressure to operate your brakes. You only get what that booster can give you no matter how much pressure you put on the pedal. Here is a formula that the manufacturers use. Drum brakes require about 400 psi while Disc brakes require closer to 800 psi. Lets say you have a single 8" diaphragm booster, here is the formula they use. 4" X 4" X 3.14 = 50.24 square inches X 8 = 401.9 pounds of force. (good enough for drum brakes) A dual 8" diaphragm booster doubles the 401.9 making 803.8 pounds of force.
A dual 7" booster uses this formula: 3.5 X 3.5 X 3.14 = 38.4 X 7 = 269.2 X 2 = 538.4 pounds of force. (good enough for drum brakes)
A single 9" booster uses this formula: 4.5 X 4.5 X 3.14 = 63.58 X 9 = 572.2 Pounds of force. (Good enough for drum brakes)
Using the formula for figuring out how much boost you get compared with the boost you need makes it much easier to choose what size booster you need. That is why when Chrysler tooled up for the front disc/rear drum, combination, they opted for the dual 8" booster and a proportioning valve in our B body cars. I believe in order to fix your problem, you need to upgrade your booster to an 8" dual diaphragm like the black unit I showed in post #6
Chrysler fixed a lot of these kind of problems before we even knew they were a problem. We only found out when we started miss-matching part that don't work together. Quiz Ehrenberg on this if you don't believe me.

I believe you. It makes sense to me as well as what Don said above as well. I will look into a booster upgrade. Thanks for taking the time to go into detail this is very helpful!

 

[Moparfiend]

Anyone knows what the industry means by “Bendix” style booster? Does it mean trapezoidal front or duel diaphragm or both?

 

[Moparfiend]

Also wondering for a front disc set up if the MC lines are facing the engine or outside left like the manual brakes? I’ve seen a mix of both.

 

[Black_Sheep]

Also wondering for a front disc set up if the MC lines are facing the engine or outside left like the manual brakes? I’ve seen a mix of both.

They can be found both ways, I have no idea why though…