Brake Bleeding Anomaly on my 66

[Steveoreno]

I installed a power disc brake kit on a 66 C-10 that I am building, the entire brake system is new, front to back, every piece. I have bled the brakes three different ways, vacuum, gravity, and even had the Warden do the old "pump and hold" . In each case the results are the same, good solid pedal with about 1 1/2" of travel until I start the motor, at which time the pedal goes nearly to the floor. I have triple checked the entire system and there are no leaks. I do have the proportioning shuttle locked in place as I didn't want it to move during my bleeding attempts. I cannot explain it, never had this happen before. Any ideas as to what would cause this? Thanks. -Steveoreno-

[Getter-Done]

Did you bench bleed the master cylinder?

Maybe the new Master cylinder is defective?

[bigmoe]

Did you move the attachment point on the pedal arm down 1" from the stock location?

[Steveoreno]

Yes to both. I'm thinking that it may be a defective master cylinder and as much as I hate the idea of replacing it I am prepared to go that route just to eliminate a variable.

[bigmoe]

https://youtu.be/dMB4l-Q4zsA?si=URM98ClnLY2sSGpP

The next time I do a bench bleed I'm going to try the syringe method.

[theastronaut]

Is the master cylinder 1 1/8" bore or 1" bore? It should be a bigger 1 1/8" bore for power discs.

If it's 1" then you could remove the booster and set the pushrod point back in the manual hole, that's how my '66 is and it brakes great with a solid pedal and they're easy to modulate at the limit of grip. We see a lot of systems that are overboosted- soft pedal and touchy modulation.

[Steveoreno]

Astronaut, that is a good point and I don't know the answer. I am going to order a 1" bore M/C from Summit and see if that makes a difference. I'll make sure to relocate the rod back to its original position and see how that works. Thank you! -Steve-

[geezer#99]

How much vacuum is your motor producing?

[Steveoreno]

I have not checked to determine how much vacuum the motor is producing. Assuming that I do a vacuum check what should I expect to see as far as pounds vacuum?

[geezer#99]

Normally 18-20 hg.
A minimum of 14 for booster operation.

[BigElittleg64]

bigmoe- what does moving the attachment point on the pedal arm down 1" from the stock location do?

[MikeN]

Originally Posted by BigElittleg64

bigmoe- what does moving the attachment point on the pedal arm down 1" from the stock location do?

Gives the pushrod more travel. I had the exact same experience as the OP, all new brakes with booster, and a very low pedal. I had to drill a new hole in my pedal arm one inch down from the original hole. Totally corrected my problem.

[MikeN]

To the OP, assuming you have rear drums. make sure the shoes aren't adjusted with too much clearance before contacting the drum.

[bigmoe]

Originally Posted by MikeN

Gives the pushrod more travel. I had the exact same experience as the OP, all new brakes with booster, and a very low pedal. I had to drill a new hole in my pedal arm one inch down from the original hole. Totally corrected my problem.

Yup same here. Once I moved the hole down an inch the problem was solved.

[Steveoreno]

Thanks guys, you have given me a great deal to work with. I'll post once I get it figured out. -Steve-

[theastronaut]

Manual brakes should have around a 6:1 pedal ratio (stock on my '66 is 6.25:1) with a 1" bore master cylinder.

Power brakes should have a 4:1 pedal ratio and around a 1.125" bore master cylinder.


Both examples are assuming you're running stock drums or the typical squarebody/D52 caliper 12" discs swap.



Just adding a booster will make the brakes soft, way too touchy, and you'll have extreme pedal travel. So when adding a booster you need to modify the pedal ratio to take away some of the leverage compared to a manual setup, which reduces pedal travel, and the reduced leverage takes away some line pressure so the brakes aren't as touchy. And then you're supposed to go up to a 1 1/8" master cylinder to again lower line pressure so the brakes aren't as touchy and the pedal travel isn't too long and the pedal isn't too soft.


With that explained... does it really make sense to add boosters to get more line pressure, but then add a bigger master cylinder that drops line pressure, and also change to a worse pedal ratio that again takes away leverage from your leg to the master cylinder which again drops line pressure? If you just stay with a manual setup you can stick with a 1" bore and see how the pedal feels, or drop down to a 15/16 or 7/8" master cylinder to raise line pressure if needed. 1" on my truck is the "just right" size with squarebody style discs and stock rear drums. The pedal travel is perfect for heel/toe downshifting, the pedal never drops any lower than the gas pedal.


Race cars have manual brakes and they stop just fine; a properly setup manual system will work better than adding a random booster and juggling bigger master cylinders and worse pedal ratios to make the brakes less touchy because you added a booster... In old cars/trucks without abs, you don't want an "easy" brake pedal; easy is impossible to modulate in a panic stop situation.

[Grounded63]

You dont add a vacuum brake booster just to get more line pressure. You get less required driver effort, for the same or better braking performance.

Disk brake calipers generally require more fluid displacement to operate effectively. ( and also more line pressure ) Than drum wheel cylinders do. Which is why you move up the a lager bore master cylinder.

Changing to a " worse " pedal ratio doesn't result in less line pressure. Your leg having " less leverage " doesn't really matter. When you've added power assisted brakes. That lets you achieve the same or greater line pressures. With less physical effort.

Race cars also have . . . Racing calipers, racing compound brake pads, racing disks, race compound tires, racing suspensions. It's not just a manual master cylinder that makes them stop just fine.

[theastronaut]

CPP says that 100 pounds of input force on the pedal with a 7:1 pedal ratio, 7" dual diaphragm booster running on 23" of vacuum, and a 1 1/8" MC bore will make 1197 psi in the lines- 704 psi from pedal ratio leverage/the master cylinder's piston area and 493 psi added from the booster's assist. Most hot rod engines don't make 23" of vacuum. If you drop the pedal ratio to 4:1 like most do in a boosted setup you'd lose line pressure with the same 100 lb input force because you've lost leverage. So in a real world setup you're not getting the full 1197 psi line pressure.

A 7/8" bore MC without a booster and a 7:1 pedal ratio will make 1166 psi with the same 100 pounds of pedal input force. So, why add a booster to end up with basically the same line pressure? It doesn't make sense when you could just add a smaller master cylinder and achieve the same line pressure.

If you have all air out of the system the pedal isn't going to drop too far with a smaller bore. Disc pads don't retract that far, and you should have working self adjusters in the rear drums so the shoes shouldn't retract far. My non-boosted pedal drops level with the gas pedal when braking, perfect for heel/toe downshifts. Its nowhere near the floor and maybe has 1-1.5" of stroke at most before building pressure. Once it builds pressure it stops dropping because there is no air in the system to compress. I have enough braking power to lock up the wheels at 80 mph, I don't need "more braking power". I already have more braking power than the tires have traction. More braking power for the same pedal input force would make the brakes harder to modulate and easier to lock up in an emergency stop situation. This is extremely important when you don't have ABS. An overboosted easy/light pedal is dangerous because it makes the wheels lock up too easily.

One of the common mods most people make when building a street car into a track car is to delete the booster and switch to a smaller bore MC for better pedal feel and easier modulation, especially on non-abs cars.

Businesses exist to sell parts, whether you need them or not. People have been led to believe that you need a brake booster to have effective brakes, but as the math shows it's just not true.

[Grounded63]

Braking systems do not only operate only at 0 pressure or peak generated pressure. There're modulated based on needed braking performance.

If you only need to generate 600psi of line pressure. Based on your given details above the power assisted driver only needs to apply 15.2 pounds of force to the pedal. While the non power assisted driver needs to apply 51.5 pounds of force to the pedal for the same braking performance. That can make a significant difference in driver fatigue/comfort in some instances.

[theastronaut]

I don't follow, unless you're saying that the amount of booster assist isn't linear compared to pedal input. Both examples have 100 pounds of pedal effort as the input with basically the exact same line pressure as the output- 1197 vs 1166, less than 3% difference.

Unless booster input to output isn't linear, the input to output ratio should stay the same between both examples. If booster output isn't linear to input, that's another reason to not have boosted brakes- the input to output should be linear for more predictable response and ease of modulation near lockup.