Converting Power Brakes to Manual

[Ol'Jim]

Early 70's Chevelles were not available with Manual Disc Brakes. They all had Power front disc/rear drum, or manual 4-wheel drum brakes.

Because power brakes use intake manifold vacuum to provide the "power", when you install a very high-performance engine, with a long-duration cam, the vacuum drops low enough to not provide adequate power boost to the brakes.

I have this situation, and so I am converting my car to manual brakes, (still has the stock-type GM rotors, calipers, rear drum brakes, etc, though most everything has been replaced in past 5 years).

I was told by several folks to use a 71 Corvette master cylinder for manual brakes, because they had front disc/rear drum, but the solid/mechanical-cam engines like the L-88 didn't provide enough vacuum to operate power brakes. I bought the 'Vette MC, and it is very close in appearance to the power brake MC on the car.

Here is my question: Can I simply bolt this MC onto the vacuum booster, and use it that way? (With no vacuum line connected to the Booster).
I realize the proper way to do this, would be to remove the booster, and mount this MC to the firewall. My problem is that the 4 mounting studs/nuts for the booster are basically inaccessible. The big-block engine with cast valve covers is right up against the inboard side of the booster, and the outboard side had hardlines, wiring harnesses, and hood hinges obstructing it. With the old MC removed, the new one will fit & bolt right in place, and I confirmed the actuator rod and pedal both travel very freely, and so really I am thinking it would just be like an extended mounting for the new manual MC. With no vacuum applied, the booster will just serve as a large mounting pad, and extension of the actuator rod from the pedal. Any reason why I can't try this out, at least until I have more time & space to remove the booster?

Thanks in advance for any advice or input you can provide.

[maxwedge]

Jim that is the same as trying to apply the brakes with the engine off, this will not work, you need a direct mount, also on some cars the manual brake pedal is different for more leverage, look into that also.

[j cAT]

long time since I worked this oldie vehicle. the brake booster uses manifold vacuum which has a check valve in the vacuum line to the booster ...so I would first check that the booster vacuum line check valve is working.

I would use a vacuum gauge and some T fittings to see that the booster holds steady vacuum with the engine running .. then shut down engine see if the vacuum holds.. if not on this you have a defect.

even with 14inches of vacuum you will get good braking..

the reason for poor braking with these rear drum vehicles is worn drums. brake drums should never be cut ... the shoes are then not the same radius as the worn larger drum. therefore only a small portion of the shoes make contact to the drum surface and weak braking.. adjusting the shoes does nothing............