1971 Mark III Brake Issues

Bill,
My Mark III had brake service performed by the previous owners at 90,000 miles, including rear wheel cylinders, shoes and hardware, front calipers, pads and hoses replaced. The system performed well and after I took delivery had it inspected for safety and everything checked out OK. At about 91K miles, it began demonstrating the tell-tale symptoms of a bad master cylinder: a slowly sinking pedal at stop signs and red lights. There was a little leakage behind the master cylinder onto the booster, and the old master cylinder looked a little too pitted inside to rebuild, so I replaced both. The booster holds vacuum and the push-rod is properly adjusted to a new master cylinder. However after bench bleeding the master cylinder and then using the two-man system to bleed the rest of the system on the car, I get a very soft pedal. It easily goes almost to the floor before the brakes “grab”. This is when the engine is running with vacuum to the booster. The pedal is very hard when the engine if off. I’ve tried three master cylinders so far, at first assuming a bad master out of the box. I’ve tried bench bleeding using the bleeder tube system and the plugged outlet ports system until there’s no more air or I’m not able to depress the piston further. Even though nothing else has changed on the brake system, I’ve checked all four wheels, the shoes are still adjusted to the drums and I find no loss of fluid anyplace. I haven’t touched the Sure Track system or pressure differential valve except to temporarily disconnect the brake lines between it and the master cylinder to make room for the booster work. The common behavior I notice each time is that when I fill the reservoirs on the bench, fluid eventually drips only from the primary outlet port, never from the secondary port. I’ve read that each port should drip by gravity alone, and that some even bleed a car by gravity. Could this be a problem with multiple master cylinders, or is gravity bleeding from both ports not necessary here? Am I missing an adjustment someplace else? Is there a particular challenge getting all the air out of the system on this particular car? I want to be thorough and consider everything before I break down and tow it in.
Bradley
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Hi Bradley –
If all of the brake work, fluid bleeding and adjustments were performed properly as you describe, your sinking pedal does sound strongly like a classic by-passing master cylinder. Since we are not the supplier or the installers of your rebuilt master cylinders, I cannot comment on their quality.
However, since you are a local customer of Lincoln Land it may be a good idea for you to make an appointment with our service dept. for a professional on scene diagnosis. Doing this could save you further disappointment in the future.
Sincerely,
Bill