I thought ever since the late 60s cars and light trucks were required to have dual reservoir master cylinders to prevent loss of brakes at all four wheels at once. My '95 F150 recently had a rear brake line rupture (rusted) and my pedal went to the stop. I really had no brakes, but the trailer brakes (good thing I was coming off I-80 in a relatively flat section of Wyoming and not the in Yellowstone Park as I was two days before or in Rocky Mountain Park where I was headed). When I looked at the master cylinder the fluid level was low throughout, whereas I would have expected to see a low level in half the master cylinder. What gives? What happened to safety? Is this part of the new plastic master cylinder design? My '84 with a metal master cylinder had dual reservoirs.

 

You are 100% right, just about all vehicles now have a common reservoir that feeds a dual brake system. There is somewhat of a baffle inside the reservoir, but it barely seperates the reservoir in half. Another thing also, I don't recall seeing a pressure safety switch on my 88 F150, just a low fluid indicator. The older vehicles had the safety pressure switch mounted to the proportion valve.
Phillip

 

In the Chilton's '87 - '93 truck manual, Chapter 9, Page 2, it explains the "Dual Circut" master cylinder operating principles. Although, it should of, at least, operated the front brakes, it should also of warned you by a brake light. I would check to see why this feature didn't function. Imagine if you were still coming down off that "hill"?

 

 

UPDATE: My former truck, an '84 F150, just ruptured a brake line today for my boss as he was driving in to work. This truck has the dual reservoir master cylinder, the old metal one with a bale to hold the lid on. He said he still had about two inches of pedal and could still stop. In contrast, my '95 F150, as noted above, when the brake line ruptured, the single reservoir drained out the ruptured line and left me with no brakes--the pedal hit the stops! What is wrong with the National Highway Traffic Safety Administration? This is the problem with government regulation. Here, for years we thought car brakes were relatively fail-safe, and the government goes and eliminates the safety requirement on us without warning. Just goes to show you, you can never trust the government. I know there still is the dual piston, but what good is it when the reservoir drains? I suppose one could retrofit the old dual reservoir master cylinder on their late model trucks, but who will bother?