Sorry to leave you hanging. Been out for a week fighting a kidney stone. Your reply was one of the last ones I read but I was in way too much pain to care at the time.
I don't think you have a brake fluid contamination problem, at least I really hope so. That is serious. There won't be any contamination in the new master cylinder from the manufacturer. They take great care to avoid that. The common consumer-caused things include engine oil, power steering fluid, automatic transmission fluid, and axle grease. Some people pour those things in not realizing they're putting the products in the wrong place, but other things that have happened are mechanics will wipe out a funnel used for engine oil, then use it to pour a five-gallon pail of new brake fluid into a pressurized brake bleeder ball. The oil residue that remains on the funnel is more than enough to contaminate a dozen brake systems. Another common cause on older cars was when we had to remove, clean, and pack wheel bearings with grease. We'd do that by hand, then when we were ready to fill the master cylinder with brake fluid and bleed the system, we found the rubber bladder seal under the reservoir cap pulled down from the normal vacuum built up over time as the brake fluid got pulled out. We used our finger tip to push the seal back into place, and even after wiping our hands on a rag, that was plenty of grease residue to cause a problem. To avoid any chance of that, most experienced mechanics wash their hands with soap and water before working on hydraulic system parts so they don't risk getting fingerprint oil mixed in.
Brake fluid is a glycol product and all the rubber parts in the hydraulic system are compatible with that. When they come in contact with any hint of petroleum product that causes the rubber to swell and get mushy. Mainly two problems result. The first is the tapered lip seals in the master cylinder grow past the fluid return ports and trap the fluid you pushed down to the wheels. That's like driving with the brake pedal pressed down an inch or two. The first clue is the trapped fluid will release right at the master cylinder when you loosen a steel line. The second clue is that rubber bladder seal will be blown up and mushy and you often won't be able to reseat it in the cap and make it stay in place.
The second less common symptom is the square-cut seal in the caliper swells so much it won't retract the piston when you release the brake pedal. It works by sticking to the piston and bending or deforming slightly when you apply the brakes. When you release the pedal it bends back to its normal shape and pulls the piston back with it, perhaps as little as the thickness of a couple of sheets of paper, but that's all that's needed to prevent brake drag. There are other physical things that very commonly happen in calipers to cause the same symptoms so those should be expected first, but I always worry about fluid contamination too since I have no first-hand knowledge of the vehicle's recent history.
There is a third place a problem can show up with a rubber flex hose, especially on those vehicles that have a metal support bracket crimped around the middle of the hose. That applies to a lot of Chrysler products, and it has happened to one hose on my '88 Grand Caravan and on my '89 Grand Voyager. One sat in storage all winter, then the problem developed, and it occurred on my '88 daily driver about two months ago. Rust builds up inside that crimp that constricts the hose. You can force brake fluid through it with the pedal but it won't release from that wimpy square-cut seal. (The fix is to simply open that crimp up a little with a flat-blade screwdriver or large pliers). It takes way longer to crawl under the vehicle than to actually do the repair. Where brake fluid contamination becomes an issue with hoses is if that problem is close to occurring anyway, the rubber will expand inside and block that hose sooner, so there's really two things working together to cause the problem.
What is hard to convince people of is there is only one proper fix for brake fluid contamination. All rubber parts that contact the fluid must be replaced at the same time, all the steel lines must be flushed and dried, THEN the new rubber parts can be installed and system can be filled and bled. Those parts include obviously the calipers, master cylinder, and front hoses, but often forgotten are the rear wheel cylinders, rear hose(s), combination valve, height-sensing proportioning valve typically used on pickup trucks and minivans near a rear wheel, and any RWAL or four-wheel anti-lock brake hydraulic controller.
You can buy rebuilding kits for wheel cylinders and calipers but they aren't a good economic option. Professionally-rebuilt units today are a lot less expensive than they were in the '80s. Also, most rebuild kits for master cylinders cost more than the rebuilt master cylinder already done, with a warranty. My additional concern is those parts are usually made from cast iron which is porous. The contamination can get into the casting, then work its way out again later recontaminating the entire system again. That is when you hear about cars that have non-stop continuing brake problems after just a few parts were replaced. The only proper fix is to replace all rubber parts at the same time. When even one contaminated part is left on the vehicle, the contamination will leach out and start the problem all over again.
Now that that's out of the way, I think you've identified a potential cause that I never stopped to think about. That's the power booster. That will not be part of a brake fluid contamination repair because brake fluid doesn't contact the rubber diaphragm, but if you notice the front brakes start to drag when you start the engine, that is the only explanation unless you have a four-wheel anti-lock brake controller. Those have three states controlled by a set of valves for each wheel. They BLOCK additional fluid flow to a wheel when the computer sees it about to lock up, then they BLEED fluid pressure off if the locking appears to be continuing, then they APPLY fluid under pressure from a small storage container called an accumulator. That cycle occurs from 15 to 30 times per second. It's very rarely heard of but if the "apply" valve were to leak, pressurized brake fluid would go to one wheel, (or commonly the two rear wheels together, especially on GM cars), and start to apply that brake. Pressure shouldn't build anyway because the "block" valve is always open to allow normal fluid flow when no wheel lockup is taking place. The normal complaint to expect is the high-pressure accumulator pump cycles on and off too often to keep the stored pressure built up.
That self-applying is not a concern if you have rear-wheel anti-lock brakes, (RWAL), which most Dodge trucks had. There is no brake fluid stored under pressure. All that system does is block fluid flow to the rear wheels, then bleed some pressure off if the wheels are still slowing down too quickly. Once the wheels get back up to speed to match the front wheels additional brake fluid comes from you pushing the pedal a little further. A concern is you could run out of brake pedal but I've never heard of that happening. Even if the pedal did go all the way to the floor, you'd just release it and push again and start the process all over. That would probably only happen if you were driving down a long, steep mountain road.
There's a couple of things you can do to identify if the power booster is the cause of the problem. First, disconnect the large rubber vacuum hose at the booster and plug it. There will be no more self-applying of the brakes. Second, with the hose connected, the booster stores enough vacuum when the engine is stopped for at least two pedal applications so you can stop the vehicle safely. After the engine has been off for a minute or more, you should feel the power assist help you push the brake pedal once or twice, and maybe even three times. I have some cars that will hold that vacuum for days but that is not necessary. If you find there is no vacuum just a few seconds after stopping the engine, it is bleeding off through the valve that is leaking and applying the brakes. That is not a common condition but it is cause to replace the booster.
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Thursday, July 4th, 2013 AT 8:43 PM