Most RV builders use MIL-H-5606 brake fluid, which has been used by many general aviation aircraft for decades. This fluid has one big weakness though - it is very flammable, and has quite a low flash point (about 215°F). Brakes that have been used are quite hot, quite possibly hotter than the flash point of the brake fluid. Thus it is possible to have a brake fire. In fact, several RVs have had brake fires.

There are other options for brake fluid. The large air transport industry moved to a fire resistant brake fluid, marketed as Skydrol. But, this brake fluid is nasty, nasty stuff - it has some major human health issues, and it must be checked periodically to ensure that it has not become acidic, which can cause damage to brake system components.

Current automotive brake fluids do not burn, so they are an attractive option. RV-8 builder Charlie Kuss is a proponent of DOT 4 brake fluid. This brake fluid is not compatible with the Buna N O-rings used in general aviation brake systems. But, it is possible to replace those O-rings with ones made from EPDM (ethylene propylene). Charlie Kuss purchased a large number of all the needed O-rings, and sold kits to many RV builders. DOT 4 brake fluid does have one big issue though - it dissolves many types of paint, so a leak will make a big mess.

The US Air Force got tired of losing aircraft to brake fires, but they didn’t want to have to change out the O-rings on brake systems. So, they developed a brake fluid that was compatible with the Buna N O-rings used in MIL-H-5606 brake systems. MIL-PRF-83282 brake fluid has a much higher flash point than MIL-H-5606 (about 450°F), self-extinguishes once the external flame source is removed, can be mixed with MIL-H-5606 fluid, and can use the same O-rings.

I agonized over which type of brake fluid to use - DOT 4 or MIL-PRF-83282. I changed my O-rings for ones made from EPDM, as it was compatible with both DOT 4 and MIL-PRF-83282 (or so I thought - another builder pointed me to info which says that EPDM is not compatible with mineral oil - I need to do some more digging on this one), and it can stand much higher temperatures than Buna N. I eventually decided to go with MIL-PRF-83282 fluid, as I was concerned by the paint damage issue. Charlie Kuss claims that DOT 4 brake fluid won’t damage modern high quality paint, but my cockpit interior was painted with MarHyde from a spray bomb.

I couldn’t find MIL-PRF-83282 brake fluid in Ottawa, but I found that Leavens Brothers in Toronto stocked Royco 782, a MIL-PRF-83282 fluid. And it wasn’t expensive - only $12 a quart. If any other local RVers want some MIL-PRF-83282 brake fluid, I have quite a bit extra.

Today I filled the brake system. I decided to fill it from the bottom, using a modified pressure sprayer, as that would push the air to the top, hopefully avoiding any trapped air in the system. Things went smoothly at first, but then I looked in the cockpit and found a huge leak. I was sure that I had tightened all the connections, but it turned out that the connection inside the landing gear box was loose. I had to clean up several tea spoons of fluid. I was very, very glad that I had not used DOT 4 fluid.

I pushed fluid in the right side until I had about a half inch in the bottom of the fluid reservoir (the highest point in the system). Then I moved to the left side and pushed fluid in until the reservoir was full (in fact, I got over zealous, and had fluid overflowing out the top).

I check the brake pedals, and as near as I can tell there is very little or no air in the system - the pedals feel fairly firm.