Brakes are nice. Having an adequate level of brake fluid is also a good thing. No arguments so far on these items. Checking or filling the brake fluid at the stock master cylinder is an absolute pain. You can reach down from under the hood and spill brake fluid everywhere. Or you can remove the access plate on the floorboard and spill your brake fluid from that direction. Even if not adding fluid, simply checking the brake fluid level is an unpleasant task that often gets ignored.
When the reservoir is empty and the brake pedal hits the floor, it makes a rather unpleasant sound. The sound itself is not so bad, but more importantly the timing of the sound is the problem. You might hear this clunking sound as you approach a busy intersection or are descending a big hill. Short of fitting explosive wheel bolts or carrying a large anchor, knowing that your brake fluid level is adequate is your best defense for stopping when desired.
I'm certainly not the first person to retrofit a remote brake fluid reservoir. Many vehicles use a similar arrangement from the factory. There are no permanent modifications to be made. Depending 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:
Here is a high angle shot with the master cylinder in the background. Rigid brake line was used so it could be bent to clear the brake pedal travel. If you use flex line, clamp it in place carefully and check the clearance around the brake pedal. Don't use ordinary fuel line as brake fluid will attack it. Where the rigid line attaches to the bottom of the remote reservoir, I used a short piece of automatic transmission cooler hose. Available at NAPA, it is rated for brake fluid, too:
Update - With the engine recently pulled for repair, here is a better view of the installation: