David,
Have you ever tried the Festo pressure regulator. JP introduced me to this great little item. Its a minature demand valve and therefore you can have a tank with air at 100 psi but the actual air pressure to the brake is constant adjusted by trial and error but very accurately. The pressure does not change even though the air tank is reducing in volume ( until it reaches the brake pressure of course). Simply run the plane out and try the brake. Adjust as necessary and its always as you set it thereafter.

Part no. pressure regulator is 267-3379 price £13.91 plus VAT Festo / RS components stock it

Regards
David