Wow. I don't know where to start. You obviously don't know much more about cars than all the other typical owners but you sure know that every mechanic is out to get you. I just finished answering another question from a naturally-suspicious person who assumed she was going to ripped off. It is very disheartening when people bring me repair bills to interpret because they found a reason to be upset. Most of the time they found out too late how well they were treated and taken care of while they assumed they had been ripped off.
The biggest problem is mechanics have very poor communication skills. That's why most shops have a service writer as a middleman to interpret and translate what they think they heard from the mechanic into something they think you will understand. They never were a mechanic, so you know things are going to get lost in translation, but that doesn't mean there was an intent to defraud.
In my city and surrounding area, we have over two dozen new-car dealers and perhaps 50 to 70 independent repair shops. Out of those, one new-car dealer and two independent shops are well-known among their competitors to be crooks, yet those three enjoy a dandy reputation because customers get fast-talked into thinking they got a great deal. You should see the looks change on their faces when I explain how they got taken and smiled while it was happening.
I find it hard to believe my city is the only one will so many reputable shops. Of course there are a few dishonest people, just like in every other profession, but it is only the auto repair industry where every mechanic and every shop owner is assumed to be guilty, even when they go out of their way to be helpful. Unfortunately, some of them live down to their customers' expectations.
Now, for your brake problem, the power booster makes it easier to push the brake pedal, nothing more. You said the pedal is going down too far, and this just started occurring. No reason to suspect the booster. You're going to bleed out the air you think is causing this. It's true air will compress and cause a low and mushy brake pedal, but how did it get there? Air can show up if the brake fluid is old AND you recently had to do a lot of hard stopping as in coming down from a mountain. Old brake fluid will have absorbed humidity out of the air, and that moisture lowers the boiling point of the brake fluid. If it gets hot enough, the water will vaporize and the resulting air bubbles will compress. That is pretty rare, and you didn't indicate any reason to suspect that type of brake fade, so lets forget the air theory. Also, even if mountain-driving was the cause, the low pedal would have shown up at that time, not hours, days or weeks later.
That brings us to your three dandy observations. There's no external leaks and the reservoir is still full. No brake fluid came out when the brake pedal was pressed. All of this points to internal leakage inside the master cylinder.
There is a trick that allows you to replace the master cylinder without having to bleed at the wheels. Loosen the steel lines a little, unbolt the master cylinder from the booster, pull it forward, then use it as a handle to bend those lines upward a little. That will keep the brake fluid from running out of them. Remove the lines all the way and remove the master cylinder.
Once the rebuilt master cylinder has been bench-bled, bolt the steel lines on finger-tight, bend them down, then bolt the master cylinder to the booster. Snug one of the line nuts and leave the other one a little loose. Have a helper slowly push the brake pedal halfway to the floor. It should take about fifteen seconds to go halfway. You'll see some air bubbles come out by the line nut. Snug that nut, THEN tell the helper to let the pedal come back quickly. Loosen that nut again and do the same thing. Now do that for the other line.
By pushing the pedal slowly, brake fluid will get pushed down to the wheels and the air bubbles will float back up to a high spot. By letting the pedal come back quickly, the brake fluid rushing back will wash any remaining air bubbles back into the reservoir.
There's two things to keep in mind. First, never push a brake pedal more than halfway to the floor. Crud and corrosion build up in the lower halves of the two bores where the pistons don't normally travel. Pushing the pedal all the way down, either when a driver is surprised by a ruptured hose, or when inexperienced people pedal-bleed the system, the pistons run over that crud and the rubber lip seals can be torn. That results in a slowly-sinking brake pedal, and it often doesn't show up until two or three days later. This doesn't apply to a master cylinder that's less than about a year old. To avoid this problem, I only gravity-bleed. I haven't used a helper since the 1980s.
Second, never fill the reservoir unless the front brake pads were just replaced. As the old pads wear down, the pistons move out of the caliper housings. That's the self-adjusting feature of disc brakes. As that happens, brake fluid fills in behind the pistons, so the level in the reservoir goes down. Experienced mechanics will not top off the brake fluid during other routine services like oil changes because they know that if the fluid is low, there is either a leak that must be addressed, or it's time for a brake system inspection, and expect to find front pads are needed. Replacing the pads requires pushing the pistons back into the housings, and that is going to push the brake fluid back up to the reservoir. If the reservoir was filled previously, the fluid is going to spill over and make a mess. Brake fluid eats paint too.
There's one more important detail everyone should know about. Do not get the slightest hint of petroleum product mixed in with brake fluid. One drop of power steering fluid, engine oil, transmission fluid, axle grease, or penetrating oil will make rubber brake parts swell up and get mushy. One of the first symptoms is a high and hard brake pedal that you mentioned. The lip seals expand and grow past the fluid return ports in the master cylinder. That prevents the fluid from releasing from the calipers. Experienced brake specialists even wash their hands with soap and water to prevent getting fingerprint grease in the brake fluid. This type of fluid contamination requires all rubber parts that contact the fluid to be replaced, and the steel lines must be flushed and dried. That is a real expensive repair. If any part is not replaced, like a wheel cylinder or your height-sensing proportioning valve, the contamination will leach out of the o-rings and seals and recontaminate the new brake fluid.
The clue to this type of contamination is the rubber bladder seal under the reservoir cap will be blown up and mushy.
When you get done, if you still have a high and hard brake pedal, and / or a wheel is getting real hot due to a sticking brake, there is an easy fix for that. Look at the front rubber flex hoses and you'll see a metal bracket in the middle of it. Rust builds up inside that crimp and constricts the hose. If you push hard enough on the pedal, you'll get brake fluid through that restriction and the brake will apply, but the fluid won't be able to release. The fix for that is to use a channel lock pliers to peel that crimp open a little. I've run into that three times already on two of my Grand Caravans.
Saturday, June 27th, 2015 AT 10:54 PM