Wrong Brake Hard Line Sizes

[Bearhawg]

So a while back I had a shop install the CPP disc brake conversion kit on my four wheel drum brake '70 C10. I've been slowly replacing most everything on the truck like entire wiring harness, fuel hoses, etc. It's a 50+ year old truck that needs some love plus it keeps me out of trouble. Next on the list is the brake hard lines. I got the complete set from Inline Tube which is specifically for disc brake conversions. Sid note: When I do the install I will have to change the orientation of the combo valve back to under the master rather than along side. As I started the process I noticed the new lines for the front brakes are larger than the lines the shop fabbed up. I got out the calipers and took measurements and found that the shop's lines were the same for front and rear, about .187in and the new Inline Tube lines for the front brakes are about .257in. Both lines from the master to the combo valve are about .235in on the CPP set up. I figured this made sense as the truck was born with manual drums all around.

Here's my conundrum. I've always had a rock hard pedal and meh braking. I've gone through the entire system multiple times , spent a lot of time listening to CPP's hold music - tested and bled the system and re-tested and re-bled the system. Do y'all think the hard line size could have anything to do with my hard pedal and "just OK" braking? I hope to get the lines installed this weekend and I'm very interested to see what happens.

What say y'all?

Thanks!!

[Davidf]

Nope

[Steeveedee]

^ What he said. Try capping off the vacuum line to the booster and see if the pedal effort changes. What do you have for manifold vacuum, btw?

[Bearhawg]

Originally Posted by Steeveedee

^ What he said. Try capping off the vacuum line to the booster and see if the pedal effort changes. What do you have for manifold vacuum, btw?

A steady 20ihHg at idle. I’ve got plenty of vacuum and the check valve on the booster is working no problems. As I’ve said I’ve been through the entire system more than once. I’ve also checked the pedal ratio. No issues there

[Lee H]

Are your pads and rotors glazed over?

[tdangle]

I'm following this as I have a brake upgrade in the near future. Per the research I've done disc in front use 3/16 line and drum in the back use 1/4 lines. I have not been able to find a definitive reason. I have also found a lot of people when converting still use the existing 1/4 lines in the front and everything works OK. Considering on running new lines in the front, moving the connection for the flex brake hose to the front of the A arm as the factory did when changing to disc in the front. there is another post about this in this forum

[67C10Step]

Can't really help a lot here but I will say I did the exact thing on my 67 you are looking to do Bearhawg. I got the complete CPP master/booster/prop valve for a good price from a guy that decided to go hydroboost. Used Inline tube hard line kit for disc brake conversion, drop spindles, and the six lug rotors. I will say I had to modify the bracket for the proportioning valve to move it under and sideways for the Inline tube kit.

Mine seems to work just fine, pedal effort and travel seem to be what I would expect. If you are going to change all the hard lines anyway it would be a perfect time to go through the system completely once more.

[Davidf]

What is the OP comparing the brake feel to? These trucks don't feel like modern trucks/cars. Pedal (assisted) effort is higher and stopping distance longer than a modern car or truck.

[BigBird05]

With all of the hard pedal posts on the forum it seams that it always comes back to either a bad master cyl or air in it.

[geezer#99]

Originally Posted by Bearhawg

A steady 20ihHg at idle. I’ve got plenty of vacuum and the check valve on the booster is working no problems. As I’ve said I’ve been through the entire system more than once. I’ve also checked the pedal ratio. No issues there

What’s the master cylinders bore size?
You might need to go smaller.

[Bearhawg]

Originally Posted by geezer#99

What’s the master cylinders bore size?
You might need to go smaller.

1.125in. I've thought about going with a smaller bore but folks way smarter than me seem to think the 1.125 is correct for my application. but I'm beginning to think you may be right. Thanks for the input!

[Caddylackn]

Originally Posted by tdangle

I'm following this as I have a brake upgrade in the near future. Per the research I've done disc in front use 3/16 line and drum in the back use 1/4 lines. I have not been able to find a definitive reason.

The answer is that rear wheel cylinders require more fluid volume to apply the drum brakes than for disc brakes. There are four moving pistons (2 per each wheel cylinder) and they have to move a longer distance to get the brake shoes to engage the drum surface. Front disc calipers do not require as much fluid to engage the brakes since the single piston in the caliper barely moves. The master cylinder reservoir on the drum brake side is usually bigger to account for the additional fluid needed. There is also a pressure residual valve on the drum brake port of the master that keeps about 7 psi residual pressure to the rear wheel cylinders so the rear brake pistons do not have to travel as far to push the brake shoes to the drums. This keeps the brake pedal travel needed to a minimum. If the car is all drum brakes or all disc brakes the brake lines can all be the same size. Having 1/4" brake lines to the disc brakes has no ill effect.

[tdangle]

Originally Posted by Caddylackn

The answer is that rear wheel cylinders require more fluid volume to apply the drum brakes than for disc brakes. There are four moving pistons (2 per each wheel cylinder) and they have to move a longer distance to get the brake shoes to engage the drum surface. Front disc calipers do not require as much fluid to engage the brakes since the single piston in the caliper barely moves. The master cylinder reservoir on the drum brake side is usually bigger to account for the additional fluid needed. There is also a pressure residual valve on the drum brake port of the master that keeps about 7 psi residual pressure to the rear wheel cylinders so the rear brake pistons do not have to travel as far to push the brake shoes to the drums. This keeps the brake pedal travel needed to a minimum. If the car is all drum brakes or all disc brakes the brake lines can all be the same size. Having 1/4" brake lines to the disc brakes has no ill effect.

Makes sense. If you look at a 71-72 C20 with disc brakes, the rear M/C fluid reservoir is bigger and goes to the front. The smaller is in the front and goes to the rear. And the M/C bore is bigger than for a C10. Am I wrong about this?