Vacuum level relationship to force applied?

[Moparfiend]

Is the vacuum level on a power brake system proportional to the power applied to the brakes? Meaning does a higher vacuum level increase the assist? Or another will a low vacuum signal lower the level of assist? Or another way to ask is will the brakes with an 20Hg signal have more assist than a 10Hg signal?

Or is the vacuum strictly a signal for actuation?

 

[R413]

engine will either make enough vacuum for the power booster to work, or it doesn't have enough vacuum and pedal will be stiff like the engine is off.

A stock 318 will have 18-19 inches of vacuum at idle. When you coast down a hill on the freeway it will have 21-22 inches of vacuum. The brake pedal feeels and acts the same in these two situations.

 

[Moparfiend]

engine will either make enough vacuum for the power booster to work, or it doesn't have enough vacuum and pedal will be stiff like the engine is off.

A stock 318 will have 18-19 inches of vacuum at idle. When you coast down a hill on the freeway it will have 21-22 inches of vacuum. The brake pedal feeels and acts the same in these two situations.

Makes sense thanks. FYI I have a modified 340 with a lumpy cam. At idle I have about 11Hg AND a 7” booster. Firm pedal works ok except the last 10 feet not much assist. Looks like an 8” booster is probably the fix.

 

[RJRENTON]

Makes sense thanks. FYI I have a modified 340 with a lumpy cam. At idle I have about 11Hg AND a 7” booster. Firm pedal works ok except the last 10 feet not much assist. Looks like an 8” booster is probably the fix.

IMO....inside the vacuum booster assembly is a proportioning valve assembly. This valve balances atmosphere pressure (ambient air pressure) against the negative air pressure (vacuum) developed by the engine and "HELPS or ASSISTS" the booster assembly to apply pressure to the master cylinder piston via the push rod and the driver's foot pedal depression. The amount of "assist" is related to the AREA of the boosters diaphragm. The FSM details the operation of this proportioning valve. Dual diaphragm boosters (Bendix)are USUALLY used with HP engines because of lower vacuum available.
BOB RENTON