ing how you mount the remote reservoir, you may not even need to drill any extra holes. This picture shows the remote reservoir on my Jeep's firewall. The reservoir is translucent and the fluid level is visible without removing the cap. It is secured with a single large harness clamp using an existing bolt on the firewall. A short length of brake fluid resistant hose is joined to a pipe running to the master cylinder:
Here is a view through the access plate in the floorboard. I made a new cap, but the original cap could be reused if the vent hole is brazed shut and a fitting attached. Note the series of shallow holes drilled around the cap, allowing the cap to be tightened with a spanner or pin wrench:
Here is the type of wrench I used on the cap. The only reason I used this type of wrench is because it was easier to fabricate the cap that way. If the cap was made from thicker stock, it could be machined with a hex head for a regular wrench or socket:
Keep in mind that this reservoir simply keeps the stock master cylinder topped off. It also is a convenient filling port. The venting of the brake system is very important. The original vent was a small hole in the stock master cylinder cap. It was easy to get water in the reservoir from road spray or submersion. The remote reservoir has the vent up high for better protection. The reservoir I chose came from the clutch master cylinder on a '79-'85 Mazda RX7. I chose this type because it has a bellows under the vent in the cap. This allows normal venting but keeps airborne moisture away from the brake fluid. This reservoir had a large barbed fitting on the bottom that I had to trim slightly.
When installing a remote reservoir, remember that rainwater will run down the firewall. I used a garden hose to double check that water will not land on the cap, even though it is sealed with a bellows.
Here is another type of remote reservoir that would work. This junkyard find is the clutch reservoir from a mid 80's Ford Ranger truck. It mounted to the firewall with two bolts. I chose not to use it as I didn't want to drill any extra holes. If you were to wander around a junkyard, you could probably find many potential donor vehicles. Remember that the remote reservoir needn't be very large as it only keeps the stock master cylinder full: