Dr. Crankshaft & the Lincoln Brakes
The Doctor and Dabney were riding, one nice summer day, in Dabney's 1958 Lincoln Continental convertible. The big (very big, at nearly 6,000 pounds!) Lincoln's rosy-red paint glistened in the sunlight and the white/red interior was spotless. As they drove along Dabney hit the brakes and was momentarily startled.
"Hey, I just lost my power assist!" he shouted. "Wait a minute, it's back."
The Doctor watched as Dabney hit the pedal a second time and the brake system brought the big Lincoln to a stop, although quite abruptly. "What's going on, Dabney," asked the Doctor.
"I really did lose the power assist there, Doc," said Dabney, "but it came back and now the car is running okay."
"We'd better take this thing into a parking lot and test it out a few times," said the Doctor.
They did, and the Lincoln's brakes went intermittent on them. The power assist worked fine a few times but then at other applications of the brakes Dabney had to push the pedal as hard as he could to slow down.
"This is a bit of a mystery, Doc," said Dabney.
"Let's look under the hood," said the Doctor.
Under the hood was a huge 430 cubic-inch engine that was still dwarfed by the enormous bay between the wheel arches and firewall. Incredibly, none of the parts were crowded together and there was enough room to get one's hands around each component. There on the firewall was the brake master cylinder, behind which was the vacuum canister.
The Doctor looked at the vacuum canister for rust, cracks or leaks and ran his hand underneath to feel for holes or indications of deterioration.
"Find anything, Doc," asked Dabney.
"No, this canister is in nice condition, especially considering its age," replied the Doctor. "I'll check to make sure the vacuum hose is tight before looking more things over."
Dabney peered around the engine compartment in an effort to find loose or broken vacuum hoses or leaking hydraulic lines, but found nothing. "I don't think anything's wrong under here, Doc," he said.
"I wouldn't be too sure," said the Doctor. "Start 'er up."
Dabney started the Lincoln's motor and followed the Doctor's instructions to repeatedly hit the brake pedal. Eventually the power boost started intermittently to fail and the Doctor listened intently as it did.
"I hear a hiss when the booster fails," said the Doctor. He then ran his fingers along the vacuum line to the brake booster and it led to a PCV valve that was inserted in a connection to the back of the intake manifold. When he touched it the PCV valve popped up and hissed, and the brake booster failed to operate. If he held it in it was quiet and the brake booster worked.
"Hmmm," said the Doctor. "Turn off the engine and let's figure out how to fix this loose PCV."
Dabney turned off the engine and went under the hood with the Doctor. "You see," said the Doctor, "this car originally had a road draft tube here in the intake but someone came along later and modified it to a PCV. That's a good thing to do, but the valve can't be loose in its fitting or a vacuum leak will occur. The PCV is intermittently slipping out of its connection and that's when the booster fails to work."
They searched in the trunk for some wire and clamps and made a temporary fix for the PCV. This would have to do until they could make the proper design changes at the shop. Meanwhile, however, there was no immediate reason to discontinue their pleasant drive in the Lincoln, so...