Technical Too much brake boost!!!

This was a kit for customers F1. Uses a 7" dual diaphragm booster. Any idea what I might expect from a 7" single diaphragm booster?
Thanks!

 

Pedal ratio is where it’s at.

 

I have never been impressed with the small diameter brake boosters.
That said, knowing which kit from which manufacturer, would be helpful. Also knowing what other parts are being used could add usable information, as is this thread will be a guessing game.

 

It’s dual diaphragm because it’s small diameter. Expect similar performance as the more typical large diameter options

 

I have a 7” dual diaphragm in my 40 with original style underfloor mount. Works very well.​

 

If you install a larger bore MC it will shorten the pedal throw and increase the effort, which effectively decreases the boost.

 

I'm aware of this. Thanks for your input but the question was related to the booster.

 

You can be impressed by this as it will put you through the windshield

 

True enough, in fact it over performs.

 

Very nice, for you.

 

Yes it will. Any thoughts regarding the original question about boosters? Pressure difference between single and dual boosters of same diameter?

 

"A single diaphragm brake booster is suitable for vehicles with four-wheel drum brakes or front disc and rear drum setups, providing moderate brake assistance. In contrast, a dual diaphragm brake booster offers greater assistance, making it ideal for four-wheel disc brake systems, as it can produce more braking power even with lower vacuum levels." This what you're looking for? Straight from the Master Power Brakes website.

 

Roughly figured, a single 7" booster with 20" of vac will provide 350 lbs of force. Dual will double that. That's in addition to the mech input from your pedal ratio acting on the master cyl piston so it's pretty easy to overboost with a high ratio pedal setup. A single 7" may well be what you need to get good feel at the pedal.

 

Thank you so much as this is exactly what I was hoping to learn. If the boost is reduced by 50% the sensitivity may be acceptable. I don't really know if a double diaphragm booster equates to 2x one 7" or not but with what you state....I'm encouraged. I think the single is somewhat shorter so will need to relocate the master or make a spacer

 

Nope. Refer to Chicken's reply

 

How does it act with no boost? Once, when I was teaching, we stripped a car down to nothing but a shortened chassis, cowl and front seat for a trainer. When you hit the brakes it would try and pitch you into the engine. Plugging the vacuum line made it drivable.

 

Real touchy brakes are often the result of the amount of free play between the booster and the master cylinder.
Most (but not all) of the rods between the booster and the master are adjustable. The reason for this is because sometimes there is enough variations between the depth of the master cylinder port, and the internal position of the rod in the booster.
Think of it this way, suppose the depth of the hole the rod fits into on the master cylinder is .060 not deep enough, when the master is bolted to the booster, it may already be applying .060 inches of braking power to the master cylinder. That would mean the amount of pressure applied to the brake pedal reacts much faster.
The other possibility could be that the hole the rod fits into on the master is .060 too deep. When the master is bolted to the booster, there is a .060 gap. When the brake pedal has pressure applied, the brake pedal has to move an additional .060 just to start moving the piston in the master cylinder, so the feeling of the driver is that the brake pedal has a long delay before the brakes are applied, and the pedal travels closer to the floor to have braking force.

You can test this very easily. Simply pull the master cylinder and add flat washers (one on each stud) between the master and the booster and bolt it back together and try the brakes. Expect the brake pedal to travel a lot farther (possibly to near the floor). See if the brake is a lot less touchy. The thickness of the washers would be a majorly large adjustment.

With the master cylinder pulled far enough away to see and make adjustments to the rod, there should be no need to disconnect the brake lines. If the rod is adjustable, there should be threads on the end of push rod that fits into the master cylinder (often the adjustment is right on the end that inserts into the master. The threads will be fine threads most likely with a lock nut between the end of the rod and where it is attached to the booster. You want to loosen the lock nut, and shorten the rod length, be aware a little adjustment here makes a huge difference. I suggest you only shorten the rod length by 1/4 TURN at a time, and then reassemble and test. The difference in the length change is done in thousandths of an inch at a time (1/4 turn or less). If your brakes are real touchy, your total adjustment may only end up a being 1/2 turn or 3/4 of a turn, sneak up on it.