David,
Before you get carried away with your buffer tank idea please read below...Especialy 8.3, 8.4 & 8.6.. Are you sure what you are planning can comply?

Geoff.

Extract from FRA build rules (29/11/05)
8.1.
Pneumatic systems should use one of the following gases: Carbon Dioxide [CO2] , Air, Argon [Ar] or Nitrogen [N2]. Other inert gases may be used but consult with event organisers for acceptability first. (The only gas routinely available from organizers of U.K. events for refilling purposes is CO2).
8.2.
The maximum pressure at any point within a pneumatics system shall not exceed 1000 psi (68 bar).
8.3.
The compressed gas shall be stored in a commercially manufactured gas cylinder of appropriate design, specification and certification. Except where the maximum storage pressure is less than 50 psi (3.4 bar). N.B. Some events may have further restrictions on bottle sizes/weights used please contact the event organiser for clarification
8.4.
The gas cylinder must incorporate a burst disc or pressure relief valve with a relieving pressure of 190 bar. Exemptions from this requirement are where the manufacturer or manufacturers agent fills the gas cylinder and applies an integrity seal / wrapping, or the storage pressure is less than 50 psi (3.4 bar). Event organisers may wish to check integrity seals prior to their use.
8.5.
Gas cylinders charged to pressures of greater than 50 psi must incorporate an isolation valve that can be operated from outside of the robot.
8.6.
Gas cylinders that do not incorporate a valve (for example: the gas is released as soon as the cylinder is screwed into it€™s mating pneumatic connection) such as found on disposable welding bottles and €˜fizzy drinks€™ machines must have an additional remote isolation valve accessible from outside the robot.