The valve is in the master cylinder and might already be tripped. Don't worry about that right now. Get the lines fixed first, fill the reservoir, open the rear bleeder screws, leave the reservoir cap just a little loose so no vacuum builds up that would impede the fluid running down, then take a break until fluid starts dripping. Close the bleeder that's flowing, then wait for the other one to start dripping. If it does, the valve isn't tripped. From then on, never push the brake pedal more than half way to the floor. That should prevent the valve from tripping.
If the second wheel doesn't drip, irritate the brake pedal a little by hand to get the flow started. If the valve is tripped, one rear brake won't have any fluid coming out. You should get a fairly solid pedal, but only two brakes will be working. That IS the test for the valve. No fluid will flow from one rear brake no matter how hard you press the pedal.
As for the no pedal, my guess is both rear lines popped a leak at the same time, or one had popped a while ago and the valve tripped to prevent fluid from going to the leak. You could tell that by excessive wear on the front brake pads on one side and little wear on the other side. The amount of difference depends on how long you would have been driving it like that.
It's also possible the master cylinder got damaged when the leak first occurred. Corrosion builds up in the bottom half of the bores where the lip seals don't normally travel with the pistons. When they finally do go all the way, those seals can be cut by that corrosion. The more common symptom on other brands of cars is the brake pedal will slowly sink to the floor when you hold steady pressure on it. GM master cylinders act differently because they use what's called a "step bore" master cylinder. The piston closest to you is larger in diameter than the second one. It moves more fluid at first to that hydraulic circuit and it pushes some fluid ahead of the front piston, THEN pressure starts to build up. They did that because they used calipers that release more than normal for lower friction and better fuel mileage, but that would leave you with having to move a lot of fluid to apply the calipers. You'd either need a bigger master cylinder or a longer pedal stroke. Either one would require a more aggressive power booster to get the job done. Instead, by using that step bore design, the first part of the pedal movement moves lots of fluid to get the calipers ready to apply, then a little more pedal movement puts the pressure to them.
If the larger seal got damaged, you will have a low pedal because the second smaller piston won't move enough fluid on its own to apply all four brakes. If the smaller secondary piston seal got damaged, the change in the pedal feel is going to be more subtle. GM doesn't seem to have a real lot of trouble with those master cylinders, so we'll cross that bridge later. If that valve is tripped, there's no reason to replace the master cylinder if that's all that's wrong with it. You'll end up with a lot more bleeding and it's entirely possible the valve will trip in the new one too.
On older rear-wheel-drive cars, the hydraulic system was split front and rear. If someone pinched off a leaking line to one front wheel, the car would pull real hard the other way when the brakes were applied. On front-wheel-drive vehicles where a greater percentage of vehicle weight is on the front, having only rear brakes working would cause the rear tires to skid and they'd find you in the next county before the car would stop. By using the split-diagonal system, you'll always have one working front brake when one system has a leak. To prevent that hard pull, they designed in a change to a non-adjustable alignment angle called "scrub radius". Basically what that means to the braking system is the left tire wants to turn to the right from braking forces instead of out to the left. The forces counteract each other instead of adding together so you might not feel any brake pull at all. Chrysler has had that really perfected a long time ago to where people don't even realize there's a braking problem unless the warning light comes on. On other brands, about all you'll see is a little wiggle in the steering wheel if you let go and watch it while braking.
If you have to reset the valve in the master cylinder, I use a rubber-tipped air nozzle to blow air into the opened bleeder screw. To prevent getting carried away, I hold it in position with one hand, then smack and release the handle with my other hand. It just takes a fraction of a second. A little pulse that would push the brake fluid a few inches in the line is sufficient. To try to work the nozzle handle with your thumb would introduce way too much air, and it would take longer than necessary to bleed it out. If you have a hand-operated tire pump with a clamp-on fitting that doesn't screw on, you might be able to make that work. Give the handle one good jab, then take it off and wait for fluid to start running out.
In all areas where you're working with brake fluid, be sure to get no petroleum product of any kind in the fluid. That will really cause an expensive headache. If you need a funnel to fill the master cylinder, it must never have been used for engine oil, power steering fluid, or transmission fluid, unless it was thoroughly washed with brake parts cleaner. Simply wiping it out with a rag isn't good enough. I have to mention that because I read about that here every once in a while.
If a tire pump doesn't work, you might try an air tank from an auto parts store that borrows or rents tools. Get the air nozzle from them too. The advantage with that is you can fill it at a gas station to a lower pressure than air compressors run at. That will help prevent introducing too much air.
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Thursday, August 25th, 2011 AT 10:46 PM
