[bigfred1958]

Check this out... I'm taking my '64 caddy to Viva next week and I just decided to tear into the brake system and give a disc swap a shot since my current front drum set up is toast.

The Swap: 64 CAddy front brake set up to 71-75 Caddy front brake set up.

The good: I have all the rotor/spindle/caliper stuff good to go.
The bad: I will have to swap master cylinders & re-bleed the system.
The ugly: I'm not sure if my drum brake booster will work with the discs and the existing front line sizes are bigger than the line sizes for the swap.

If anyone can help... Need Help with the following:
- The caddy disc boosters are ~2+" bigger in diameter & ~1+" bigger in depth than the drum boosters, making the disc booster a tough fit. Is there a reason for this? Do discs require much more boost? Will I be ok with using the drum booster???

- The front drum lines are a bit bigger than the front disc lines. Can I get away with using adapters? Or is there a reason for the small line size on the Discs?

- How can I check my existing drum booster? The brakes are so f'd up that it's hard for me to tell if my current braking condition is due brake parts (shoes, drums, cylinders) or if it's due to a bad booster? Is there a sure-fire way to check a brake booster?

Sorry for the length, any help is appreciated!

[Triaged]

I would replace all the brake lines just because they are so old.

One way to test a booster is to see if it will hold vacuum. It should hold vacuum enough to provide for 1-3 pushes of the peddle after the engine is turned off before it becomes rock solid.

[bigfred1958]

Thanks,
I'll go out and check my booster.

[POWERBRAKEBOB]

Disc Brakes Require More Volume Because You Go From A 1.125" W/cyl To A 3" Cyl, And Require Usually 1200psi To The Calipers. This Is Because Drum Brakes Self-energize, I.e. Wrap-up In The Drum Jamming Them In. Discs Have No Self Energizing Action.this Is Why Discs Do Not Lock, And Drum Brakes Do. Drum Brake Boosters Had 1" Masters And A Booster Set To Approx 600psi. ,the Lock-up Point Of Drum Brakes. The Discs Need 1200 Or More. That Is Why A Proportioning Valve Is Required On Rear Drums. The 1200 Psi Going To The Discs Will Over-ride The Drums And Slide Tires At 600psi, And The Discs Are Only Half Up To Pressure. That Is Why The Dual Diapragm Booster Was Invented. The Larger Bore Master Needed For Discs (1.125") On A Drum Brake Booster Lowers The Line Pressure To Approz 450psi. You Are Then Going The Wrong Way. The Dual Dia Booster Or Hydroboost Systems Are Able To Apply More Pressure Against The Master Pistons To Raise Pressure Up To The Needed 1200psi. Now, With The Larger Camshafts, Vacuum Lowers Inside The Booster. The More Inches Of Dia. Area, Multiplied By Every Inch Of Vacuum Available Gives The Power Output Of A Vacuum Booster. For Example. A Booster With 100sq Inches Of Dia. Area Multiplied By 20" Of Vacuum Theoretically Would Push 2000psi Against The Master. Lower The Vac. To 15", Lose 50% Of The Boost. Lower The Vacuum To 10" And You Have No Assist. Doesn't Add Up? The Reason For That Is Power Brakes Have A Different Pedal Ratio Than A Manual Pedal. (the Angle Of A Chevelle Booster For Example) A Manual Pedal Has A High Ratio, Or "pry Bar" Effect On The Master. A Booster Only Needs The Control Valve Cracked Open To Begin Applying The Brakes. So, In Answer, You Need A Dual Dia. Booster And An 1.125" Master, With Prop Valve To Complete Your Conversion. We Have Installed Hydroboost On Those Cars If You Need The Room.

[willys36@aol.com]

It doesn't matter what size line you use in a brake system. Has zero affect on function. Just make them fit w/ adaptors.

[bigfred1958]

Thanks for info on the lines. I'll just use adapters.

From what I've read it sounds like I need to ditch the existing drum/drum booster & put on a disc/drum booster & Master Cylinder.

I have a brand new booster/master cylinder for a '58 chevy disc/drum conversion... Other than mounting & push-rod issues, is there any reason this won't work?