FYI- This is from DriVer's post back on 12/22/04 on the brake fluid subject:
First let's deal with the lumpy brake fluid. I spoke with a tech that had 25+ years in the business coming over from GM some 5 years ago when Workhorse started up. In his recollection he hasn't seen lumpy brake fluid however he offered that occasionally brake fluid may present a scum or a skim coat on the surface of the fluid in the master cylinder. Contacting Bosch about this issue in the past, Bosch offers that this film is derived from the preservative that is used when all the brake components are brand new. The preservative mixes in with the brake fluid and what you see is an innocuous film on the surface of the fluid.
Bosch stated that this issue presents no short or long term problems and not to worry about it.
Continuing on with our conversation about brakes I was given a primer about brake fluid even so I could understand it. There are currently 3 types of brake fluid, DOT3, DOT4 and DOT5. All Workhorse gas powered chassis ship from Union City with DOT3 brake fluid. When the chassis are shipped to the OEM and they section the chassis, the hydraulic system for the brakes also gets reworked. Most often the manufacturer or 3rd party chassis converter will remake the hydraulic system and they should use DOT3 however they've seen where they have used DOT4 on top of the DOT3.
This in of itself is not a bad thing since both the 3 and the 4 are glycol based fluids, DOT4 having a higher operating temperature rating. The problem happens when Mr. Owner goes to his local parts store and buys brake fluid that is labeled Racing Brake Fluid. The Racing Brake Fluid is designed for racing applications and as such has an extremely high temperature rating.
The problem with DOT5 is that it is NOT compatible with your braking system and could cause a bunch of problems. I will name a few here if I can remember. First off DOT5 is a silicone based fluid and it's not mixable with DOT3 & 4. If you try and mix silicone and glycol they will separate out and your braking system may not even transmit the force required to operate the calipers. DOT5 will also not mix with water. Water will separate out of DOT5 find a low spot or a nice metal surface to attack and create rust and corrosion and then the troubles begin. When moisture gets into DOT3 & 4 it mixes with the fluid and will eventually turns dark in color and will require flushing however unless there's a lot of water it shouldn't hurt anything.
DOT5 used in a modern passenger vehicle will also disable the ABS unit. In order to restore your braking system after the introduction of DOT5 the system would have to be completely flushed repeatedly. Workhorse will cover the warranty service to repair your hydraulic braking system if you introduce DOT5 to your master cylinder much the same as the coach builder would warranty the grill work if you hit a telephone pole. It ain't gonna happen.
You can use DOT3 and DOT4 and that's it. The reason why this was brought up is because they have seen a few cases where people tried to improve the performance of their fluids only to find out that DOT5 is not compatible with the hydraulic system as installed on our motorhomes.