Larry,
The BVM "Smooth Stop" brake valve is a very worthwhile investment. They list for $74.95.
Call Patti at 407-327-6333
They only utilize 1-2 PSI from the storage tank per application. They are very easy to set-up, and are a welcome addition to any jet. They require a micro servo such as a JR351 to operate them.
I use one air tank in my BobCat for both the retracts and brakes. I pump the tank to 110-120 PSI, and routinely have around 60-70 psi left after flight. The "Smooth Stop" requires 40 psi minimum to function, so pressurize your tank just before taxi to have the maximum amount of air available.
I use a mixer to hook the brakes to down elevator, and use Aux 3 as well for brakes. I activate the brake/down elevator mixer with the gear down switch. The brake servo does not operate when the gear is up.
The best set-up, is a seperate tank for the brake system. You can do this if the space is available. The extra air tank only weighs about an ounce.

I don't think that there is such a thing as brakes that are too good. Most overrun accidents are caused by the lack of reliable brakes.
An expensive airplane can be turned into worthless junk when you try to cut corners on important accessories.

Harley Condra
BVM REP
RAM REP
Buy American!!!!!!