The booster should not be the issue. It only makes the brake pedal easier to push. I suspect this is a custom setup as there were no dual master cylinders in 1938. My first question is what original application is the master cylinder from? If it is for a front-wheel-drive model, it is likely of the split-diagonal design. That puts one front brake and the opposite rear one on the same circuit. That is a bad choice for your car. The idea is if you lose half the brakes on older rear-wheel-drive models, you lost about 70 percent of your stopping power if the front system sprung a leak. With more weight, by percentage, on the front of front-wheel-drive models, the front brakes do about 80 percent of the stopping. If a hose popped, leaving you with just the rear brakes, the rear wheels would lock up, you'd keep skidding down the road, and they'd find you in the next county.
The split-diagonal system helps because if part of the system fails, you'll still have 50 percent of normal stopping power with the one working front brake.
Where the problem comes in is if you were to block fluid flow to one front brake, or drive a car the fellow had disabled the caliper on, you'd find applying the brakes would tear the steering wheel out of your hand and be very impossible to drive. As part of Chrysler's development of the split-diagonal system, they modified key front alignment angles so when braking, the left wheel wants to pull to the right pretty hard. That pull offsets the brake pull to the left. In fact, they had that so well perfected that most drivers never noticed a braking problem other than the red warning light turned on. For most other manufacturers, the most you'll notice is a slight wiggle in the steering wheel, but the vehicle will still stop in a straight line.
If your car is using the original steering and suspension system, the alignment angles are designed for use with balanced left and right braking power. You may not notice a problem when it's working normally, but if one system develops a leak, and you apply the brakes with just one working front brake, the steering wheel can rotate hard enough to break fingers. You'll need to hold it straight with both hands.
If you do have a standard front / rear master cylinder, is it new or used? If it's used and more than roughly a year old, it is very common for crud and corrosion to build up in the lower halves of the two bores where the pistons don't normally travel. Pushing the brake pedal all the way to the floor, in particular when pedal-bleeding with a helper, runs the rubber lip seals over that crud and can rip them. That results in a slowly sinking pedal, and often takes two or three days to show up. Eventually one or both systems stop working. That might be the point you're at now. One possible way to check, if you have the right kind of reservoir cover, is to remove it, then watch the fluid when a helper pushes the brake pedal rather vigorously. You should see two spurts of fluid going straight up. The pistons / lip seals push that fluid out through the "replenishing ports", then, once the seals have moved far enough to pass those ports, the fluid ahead of them is trapped and gets pressurized as the pedal moves further. When one of the lip seals is torn, that spurt of fluid will be very weak or nonexistent. Be careful when watching for that. It's not unheard of for a brake fluid spurt to splash onto the bottom of the hood if the pedal is pressed very fast and hard.
If you do have the right style of master cylinder, and the seals are likely not ripped, be sure the front brake section is feeding the front brakes. With the older cast iron master cylinders, the larger reservoir section was for the front brakes and the smaller section was for rear drum brakes. As rear shoes wear, there is a mechanical means of keeping them in adjustment, so the brake fluid level never goes down in the reservoir. Front disc brakes are different. As the pads wear and become thinner, the piston moves out of the caliper housing, then brake fluid fills in behind it. Most of the extra fluid in the reservoir is used up by the time front pads are worn. That level goes back up if the piston is pressed back in to make room for the new, thicker pads. That's why we never top off brake fluid like we do with other fluids during routine services like oil changes.
A less-common cause to consider is what happens if the brake fluid was contaminated with a petroleum product. That will cause rubber parts to swell and become mushy. The first symptom is the lip seals in the master cylinder will grow past the fluid return ports, (replenishing ports), and block them. When the brakes get hot during normal driving, that heat migrates into the fluid, causing it to expand. Since the ports are blocked, the fluid can't flow freely back into the reservoir. Being trapped with increasing pressure, the brakes self-apply harder and harder. One clue here is the brake pedal will be higher and much harder than normal. If you haven't been able to drive your car yet and warm the brakes, the grown lip seals will prevent new fluid from running down from the reservoir to fill in ahead of them. Pedal-bleeding will be ineffective, and if gravity-bleeding, (which is the only method I use), the lip seals will prevent fluid from running down to the bleeder screws. Gravity isn't strong enough to overcome them.
My last thought for now pertains if you're using a master cylinder from a GM model. Front-wheel-drive master cylinders, and possibly other models, have an internal valving system to block fluid flow in a circuit doesn't build the same pressure as the others. In other words, it has a leak. Imagine a teeter totter inside, with plugs that can move and block one port. That stays balanced when both systems develop equal pressures. If a leak develops in one circuit, that teeter totter moves to block that port so no fluid is lost. The problem is that valve is very easy to trip just by pedal-bleeding with a helper. Once that happens, it will not reset on its own, and you won't get fluid flow to two wheels. It's even possible to trip that valve by hard bench-bleeding a new master cylinder.
The only way I have ever found to unseat that valve is to loosen the cover on the reservoir, go to one of the wheels that isn't flowing any fluid, loosen that bleeder screw, then give it a quick, short burst of compressed air. More air is definitely not better. You don't want to push fluid all the way back into the reservoir, then have to bleed that air back out. All you need is a tiny puff to move the valve off its seat.
Let me know if any of these suggestions pan out, then we'll try to figure out where to go next.
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Thursday, December 19th, 2024 AT 3:25 PM
