I don't know about the Summit booster, but I've had no problems with aftermarket Lockheed booster like the one sold by Skandix (and they have a good price). This is a separate unit that doesn't connect mechanically to the pedal or MC in any way -- it works just like the separate boosters that were options from Volvo at the time.

As Mr. DeWolfe correctly points out, though, all any booster will do is make the pedal effort lighter. If you can lock the brakes with the system you're running now, you can't have "more" brakes than that. You need grippier tires first, then revisit how to lock those up too.

To answer your question about the Summitt parts, you would need to measure carefully to ensure everything will fit. You might want to mock up a cardboard "vacuum canister" and see how far out you would need to mount it to clear the clutch master. Be sure to check that you don't foul the distributor or inner fender too. It looks like the biggest bracket kit they have just might clear the clutch master cylinder.

If you do go to a vacuum can, you might want to reduce the leverage on the brake pedal; it just might be really touchy if you leave it.
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Mr. Shannon DeWolfe -- (I've taken to using Mr. because my name tends to mislead folks on the WWW. I am a 51 year old fat man ;-) -- KD5QBL

As TomTom pointed out, brake boosters ease pedal effort but do nothing for the braking capacity of the system. However, his local chap needs to rethink brakes if he advocates bigger is better when it comes to brake disks. The diameter of the disk has no effect on braking power. Brake pad and shoe street compound materials have a minimal impact on braking performance. The μ (mu) for street pads is typically < 0.3. Plug that number into the friction equation and you will see that maximizing system pressure produces the most gain.

Here is the equation for creating pressure in the system:

P = F/A

where,
* P is the pressure generated in the system,
* F is the force applied to the piston in the master cylinder, and
* A is the area of the piston in the master cylinder

If F goes up, so does P. The only way to increase F is to increase the leverage of the pedal.

If you consider the vacuum booster as an enhancement to F, I suppose you could make the assertion that it will improve braking. But maximum system capacity is unaffected.

Consider A: As A goes down, P goes up. A smaller diameter master cylinder will increase the system braking capacity. If you go this route, you have to make certain the stroke of the chosen master cylinder is sufficient to supply enough fluid to the system.

At the wheels, the situation is reversed:

F = P x A

P is system pressure, whatever the master cylinder is delivering. A is the area of the caliper piston or drum cylinder. F is the force applied to the brakes. You can see as A increases, F does too. Bigger wheel cylinders will produce more braking power.
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Mr. Shannon DeWolfe -- (I've taken to using Mr. because my name tends to mislead folks on the WWW. I am a 51 year old fat man ;-) -- KD5QBL

Bear in mind that a brake booster will not actually make the brakes better, it just reduces pedal-pressure somewhat. My local chap, who fits out competition and rally Amazons, usually recommends uprated pads and discs to cope with a power-upgrade.

Tom