Hi guys. I have a couple of comments that will help. First, as Ken has been mentioning, you need a scanner to command the computer to open two valves so trapped pooled air can be expelled from two chambers, then be pushed out to the wheels. Going to a mechanic without that scanner is like going to a surgeon who doesn't have a scalpel.
Next, the glaring comment was when you said it only bleeds from one front brake and the opposite rear one. You can put as many new master cylinders on as you want to, but you'll keep having the same problem. This sounds like a version of GM's master cylinders. Think of a teeter totter inside and on each end is a valve that blocks one of the lines leaving that master cylinder. That teeter totter stays balanced, or neutral, with no ports blocked, as long as equal pressure is built up in both halves of the hydraulic system. When one side builds more pressure, such as when a leak develops, or you try to bleed manually with the pedal and a helper, or when you just pump the brake pedal to run the pistons back out of the calipers to adjust them, one side always builds pressure first, then that teeter totter trips to block the port with lower pressure. In that way you don't lose brake fluid to a leak.
Normally, having just one front brake will tear the steering wheel out of your hands hard enough to break wrists, but when this split-diagonal system is used, the steering and suspension geometry is modified to overcome that. Chrysler had that so well perfected that the only way to know there was a problem was through the red "Brake" warning light. On most other brands, the most you might see is a tiny wiggle in the steering wheel when the brakes are applied, but you have to look for it.
To add to this misery, almost all service manuals say to push the brake pedal all the way to the floor when pedal-bleeding. Those instructions don't take into account what happens after the vehicle is more than about a year old. Crud and corrosion build up in the lower halves of the bores in the master cylinder where the pistons don't normally travel. By running the pedal all the way down, the rubber lip seals run over that crud and can be ripped. That commonly results in a slowly sinking brake pedal, and that often takes two or three days to show up. You don't have to worry about that yet with your new master cylinders, but pushing all the way while bench-bleeding may also trip that internal valve. I've had good luck by never pushing the brake pedal more than halfway to the floor, and I don't get too involved with bench-bleeding either. Forget about the last few remaining air bubbles. Once the unit is installed, those bubbles aren't going to go down to the wheels. If you push the brake pedal slowly, the bubbles will float back up, then get washed into the reservoir with the returning fluid when the pedal is released. Within a few normal stops, all the air will have been expelled into the reservoir.
The same thing works when replacing the master cylinder. There's no need to bleed at the wheels. The air is up on top. Why push it all the way down to the wheels, especially when much of it is going to get trapped in the ABS hydraulic unit.
As for that valve that's tripped in the new master cylinders, the only way I've ever found to solve that is to open either one of the bleeder screws that won't flow any fluid, then give that one a very short blast of compressed air with a rubber-tipped nozzle. It takes just a tiny puff of air to unseat the valve in the master cylinder. You don't want to inject so much air that it takes forever to bleed it back out. I like to give the nozzle a quick karate chop with my hand. Shortly after that, fluid will start flowing from gravity. Leave the cap loose on the reservoir when doing this.
Some service manuals from GM say to bleed by starting at a certain wheel, but that doesn't make sense. If that valve won't trip if you start bleeding at the right rear, for example, it isn't going to trip when that circuit develops a leak either. That makes the whole point of that valve useless. I always start with the wheel I'm standing closest to. Gravity-bleeding is usually the only method I use, but once in a while it's necessary to "irritate" the brake pedal a little, by hand, to get the flow started. Again, leave the reservoir cap loose so no vacuum builds up that would impede that flow.
Hope that helps to reduce the frustration.
Tuesday, July 29th, 2025 AT 11:29 PM
