500psi sounds right but 1200psi does not.
I'd take the MC apart and double check the bore and do the math.
Use the gauge to conform you get the same pressure at the caliper with and without the Prop Valve.

 

500psi sounds right but 1200psi does not.
I'd take the MC apart and double check the bore and do the math.
Use the gauge to conform you get the same pressure at the caliper with and without the Prop Valve.

Its supposed to be 1200 in front 600 in rears. I'm only getting the 500 in front with prop valve, without prop valve, at master cylinder and at calipers.

 

The M/C pistons for the front and back brake circuits are the same diameter. Common sense should tell you that the pressure will be the same in both circuits. I don't know were you are getting these pressure numbers, but there is no difference. Also note that the 1967-70 A-body disc brakes do NOT use a proportioning valve. If this is a factory disc brake car, there should be a metering valve bolted under the master cylinder, but this only serves to delay application of the front brakes for a fraction of a second to allow slop in the rear drum linkage to be taken up. If this car has been converted to disc brakes, then all bets are off.

Your problem is the 7" booster. Even a dual diaphram 7" booster has about 20% less diaphragm area than the stock 11" single diaphragm booster. You need either at least an 8" dual diaphragm booster or a hydroboost. Also consider going to a 1" M/C instead of 1 1/16".

 

Its not a common sense issue. After all the front brakes perform most of the stopping functionality. And according to LEEDS Brakes, Wilwood, and any number of manufacturers, that sells brake parts, or the pressure testers, as well as a bunch of articles Ive found to confirm front brake pressure, its 11-1200. I wish it was 500, the same as the back cause then Id have brakes. Thanks though.

 

Line pressure is ONLY a function of M/C piston diameter, booster size, and pedal ratio. If the pistons for the front and back circuits have the same diameter, the line pressures will be exactly the same. THAT'S the common sense part. You get more brake pad pressure at the front by using a larger WHEEL cylinder diameter. The piston in the caliper should be 2.95" diameter, as compared to the 7/8" or so at the rear wheels.

 

Then all of these companies must be wrong. I'll let them know..... Thanks again

 

This is why you should have stayed awake in math class. Pounds of force on the pushrod into the master cylinder divided by the square inches of the M/C piston give you pounds per square inch. The only way to change that is to change the pounds on the pushrod (ie, more booster diaphram area) or different piston size (fewer square inches). Math is your friend. I've found that most aftermarket brake companies do NOT provide correct info, they just want to sell the products they have on the shelf.

 

I'm very good at math thank you. I doubt companies are selling products knowing they're insufficient, especially when it comes to brakes, just to sell parts, especially when it comes to Leeds, Wilwood, or any reputable manufacturer. It would be much easier for them to say "Oh you have 500psi at the front? Perfect! See ya..."......

 

So you seek advice and yet shoot it down? okay

If you call LEEDS and get advice, follow it instead of here arguing here. Papers are wrong pretty often. The internet is wrong most of the time.
200psi will stop the car, 500 will stop it real fast and 650" should lock it up.

For what it's worth, The booster doesn't change pressure, thats the hydraulic system working, The booster just eases the pedal effort to get to the desired pressure at the caliper.

 

I'm not shooting it down. I don't need someone I should've stayed awake in math class or insinuate I'm lacking common sense. I did call LEEDS cause that's where I bought all the parts from. They're the ones that told me I should have 1000-1200 psi at the calipers and half that at the drums. While I was waiting for them to get back to me I did a little research and that's what I found; the same thing on the Wilwood site and articles. Not shooting anything down, but I was looking for help not condescending comments.

 

Up to 1200psi in a stand on the brake pedal PANIC stop, lock up all four wheels type deal. MAYBE.
I do just fine at 500psi. usually just 100 to normal day to day stop.
While quite a bit lighter, even 300 is all I need to stop in my race car.
I just don't see an issue at 500psi. It should be stopping.
I think what you have going on is a low pedal ratio and not enough assist for that ratio.
When LEEDS said "not to exceed 4:1", I wonder if that should have read as the minimum. As in 5:1 or 6:1 is better.

 

Up to 1200psi in a stand on the brake pedal PANIC stop, lock up all four wheels type deal. MAYBE.
I do just fine at 500psi. usually just 100 to normal day to day stop.
While quite a bit lighter, even 300 is all I need to stop in my race car.
I just don't see an issue at 500psi. It should be stopping.
I think what you have going on is a low pedal ratio and not enough assist for that ratio.
When LEEDS said "not to exceed 4:1", I wonder if that should have read as the minimum. As in 5:1 or 6:1 is better.

I’m not exactly sure what the problem is. I’m just going by what they’re telling me.

 

11" diaphragm is 95" square inches.

2 7" diaphragms is 38.48x2=77 sq inches

2 8" diaphragms is 100 sq inches

Joe is pointing you in the right direction. Not enough assist for the ratio.

 

I’ve questioned whether or not a 7 inch dual diaphragm is enough and the said that’s plenty, that I don’t need an 8 inch. I’ve also questioned if I should go to a smaller bore size. Currently at an inch and an 1/8th. They said no, anything smaller would result in higher pressure but at the cost of having to push the pedal to the floor and feeling spongy. Again I’m not questioning anyones intelligence and not trying to offend, I’m just conveying what they told me. So I’m my mind I’m now wondering if there’s something wrong with the distribution block down on the frame. Although that’s new as well. I don’t know….

 

I mean, I hear ya, I'm just going by experience and that experience is telling me the 1200 sounds fishy and when ever humans are involved, things get fishier.
You bypassed the block already correct? Verify your pressure at the MC, the block and at the calipers. Do all four and any drop in pressure, indicates your issues.

 

I mean, I hear ya, I'm just going by experiance and that experiance is telling me the 1200 sounds fishy and when ever humans are involved, things get fishier.

I also did check the pedal ratio. That was about 3.7. I’m sure there’s some wiggle room, I was up under the dash with a tape measure.

 

Yeah I wasn't trying to say you were wrong there, the system is a balalance of effort and assist. One of those is not in check.

 

I'd THINK, 500 front and back would get the job done. It's a heavy car, but even at 500psi it should be stopping okay, maybe not great, but far better than standing on it at 10mph and it not working.

 

So I have a 57 C3100 pickup with the 7" booster and now disc ft and rr. Leed told me I should go to as large of booster I can fit because the 7" just doesn't have enough capacity to compress the brakes for more than one push. I also have a 1" bore MC. One question I have is does the pump shut off after a few sec while you hold the pedal down, it should.

 

Yeah and to get that 500 I am literally standing on it.

 

Are you sure you have enough pedal stroke to fully depress the MC ?

 

I’m not sure. I know the pedal ratio is where it’s supposed to be, according to LEEDS. I also know the pedal travels about 2 1/2 inches. The pedal seems to be at its highest possible position without/before stepping on it. LEEDS also told me the rod coming out of the front of the booster that goes into the back of the master is preset at their facility. I asked them if that needs to be adjusted perhaps cause I see it has a jam nut on it. But they said no.

 

This is from a hotrod magazine article. I found similar information elsewhere ..

 

Thanks....Thats what Im getting, 500psi all the way around the car, but should have 11-1200 at the front. I know the pedal ration is correct, and I have a bandit vacuum pump hooked up, I sent Leeds back their master and booster, both which they said tested fine. Maybe the 7 inch dual diaphragm isnt enough. They tell me it is so who knows.......

 

How are you calculating brake pedal ratio?

 

basically dividing the length between the middle axis of the brake pedal to the top pivot point by the length between the pushrod going to the booster and the same middle axis as the brake pedal. In my case it came out to be 3.7-ish.

 

Just doing some math on it.......Assuming a few things from the post.

1 1/8" MC bore is .994sq inches of area
Pedal Ratio is given at 3.75:1
At 50lbs pressure on the pedal, as if a 50lb weight is strapped to the pedal, that's 188lbs fluid pressure at the MC.

50lb of leg pressure is a good push, especially just using the ankle, in order to get to you proposed 1200lbs, you would need 318lb of leg force at 3.75 ratio on a 1.125 bore.
318lbs......That's a really hefty leg press!

See what I mean when I said the 1200psi all over the internet is wrong?

You need more ratio or more assist. A lot more ratio, or a lot more assist.
Just for kicks.....
At your usual 6:1 ratio and 50lbs of leg force you get 301psi at the MC.

Either way you go, you have some hydraulic pressure at the brake pad interface not calculated here. I don't know the piston size of your calipers.

 

Just doing some math on it.......Assuming a few things from the post.

1 1/8" MC bore is .994sq inches of area
Pedal Ratio is given at 3.75:1
At 50lbs pressure on the pedal, as if a 50lb weight is strapped to the pedal, that's 188lbs fluid pressure at the MC.

50lb of leg pressure is a good push, especially just using the ankle, in order to get to you proposed 1200lbs, you would need 318lb of leg force at 3.75 ratio on a 1.125 bore.
318lbs......That's a really hefty leg press!

See what I mean when I said the 1200psi all over the internet is wrong?

You need more ratio or more assist. A lot more ratio, or a lot more assist.
Just for kicks.....
At your usual 6:1 ratio and 50lbs of leg force you get 301psi at the MC.

Either way you go, you have some hydraulic pressure at the brake pad interface not calculated here. I don't know the piston size of your calipers.

I think the caliper piston size is 74 or 77 mm maybe? For power brakes, they recommend a pedal ratio not to exceed 4, and on manual, I think it was 6 or 7. After talking to the support person again at Leeds he says 500 at the MC is fine, but should be 11-1200 at the calipers. He initially told me I should get that at the MC, so thats why I made a line directly to the MC and put the gauge on the far end of it..........

So Im getting the same 500 at the calipers Im getting at the MC for the fronts (and the rear drums as well). Does the pressure increase somehow at the calipers? Appreciate the help.

 

OK, sounds right

 

Yes, the hydro/mechanical advantage of hydraulics.
74mm is 2.91 piston diameter. That isn't right. 77 is 3.03" piston, that isn't right either.

Joe says 2.95" or 75mm.
Assuming he correct, and I believe he is,

Caliper area is 5.94 inches, MC area is .994. 500psi of pressure is about 2990lbs at the pad to rotor pressure.

 

Yes, the hydro/mechanical advantage of hydraulics.
74mm is 2.91 piston diameter. That isn't right. 77 is 3.03" piston, that isn't right either.

Piston Size (OD) (in): 2.94
Piston Size (OD) (mm): 74.55

 

I'm just guessing , but I'm old & don't care if people think Im stupid ! I think there's a problem in defining whether its applied pressure or pressure applied . stay with me . If you apply 500 psi to a 3" sq. Surface then you should be applying , 1500 psi to the object being moved , I believe thats whats called hydraulic advantage ,No ?
Edit , to check your results , use a vehicle you are sure is functioning properly , hook your gauge up on your daily driver , see what readings you get there !

 

I'm just guessing , but I'm old & don't care if people think Im stupid ! I think there's a problem in defining whether its applied pressure or pressure applied . stay with me . If you apply 500 psi to a 3" sq. Surface then you should be applying , 1500 psi to the object being moved , I believe thats whats called hydraulic advantage ,No ?
Edit , to check your results , use a vehicle you are sure is functioning properly , hook your gauge up on your daily driver , see what readings you get there !

That is correct.

 

Yes, the hydro/mechanical advantage of hydraulics.
74mm is 2.91 piston diameter. That isn't right. 77 is 3.03" piston, that isn't right either.

Joe says 2.95" or 75mm.
Assuming he correct, and I believe he is,

Caliper area is 5.94 inches, MC area is .994. 500psi of pressure is about 2990lbs at the pad to rotor pressure.

you need more assist to reduce the leg pressure.

 

With this Bandit vacuum pump im getting 21 inches vacuum going to the booster. Question - Im using a 7 inch dual diaphragm booster...... will going to a bigger booster, 8 or 9 inch decrease the amount of leg pressure even though the vacuum may stay constant at 21 inches?

 

So if the OP is reading 500 psi at the caliper inlet , the 3" caliper piston is indeed delivering 1500 psi of pressure to the pad . To " read" that pressure would require a different type of gauge , like a valve spring checker , correct ?

 

That correct. At that point it's 1500lbs of pad pressure against the rotor, you measure that with a scale between the rotor and pad.
Stick your finger in there and feel it.

 

Lol