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The Audi Doctor
The Audi Doctor, Technician
Category: Car
Satisfied Customers: 2428
Experience:  Over 30 years experience, 17 years with Audi, Factory trained
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Hello!I have a 1975 Lincoln Continental Town Coupe 460

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Hello! I have a 1975 Lincoln Continental Town Coupe 460 cui. I have a problem with brakes. It started with low steering pump fluid (leaky steering gear) but when I added fluid brakes started to make problems. Pedal woul just slowly go to the bottom when depressed and brakes wouldn't work. Only if i press the pedal quickly I get the pressure and the car brakes. So, sometimes it works, sometimes the pedal goes to the bottom... Is it the master brake cylinder?

Hello,

Thank You for choosing Just Answer and Pearl.com for your query. Yes, I can help you.

Yes, I believe you are “spot on” with your diagnosis. The Brake Master Cylinder is likely failed. This is a “classic symptom that I have been dealing with since I started working on cars back in 1974. The correlation between the two, steering fluid and sinking brake pedal, is unlikely however.

What happens is that over time the rubber plunger cup seals in the master cylinder harden over time. This allows the fluid to bypass the seal under light applications and seal under “quick” pressure.

A cup seal in a master cylinder works like this; When you press on the brake pedal the cup slides down the bore of the MC with the inside of the cup pressurizing and displacing the fluid. The fluid in turn goes down the brake lines and pushes the fluid down the lines and pushes the caliper pistons out to squeeze the brake discs, and pushes the wheel cylinders out to apply the rear brake shoes.

What has happened in the case of your Town Coupe is that when you apply the brakes gently the brake fluid bypasses the cup seal, but when you “stab” the brake pedal the pressure on the inside of the cup seal forces the walls of the cup seal to seal against the bore of the MC and the brakes appear to function as they should.

I suggest you have the master cylinder replaced as soon as possible. When safety is a concern I always have to recommend that you tow the vehicle if it must be moved prior to repair. You never know when it will “give up the ghost”.

I hope I have answered your questions and addressed your concerns. Should you have further questions on this matter, please do not hesitate to ask. I want you to be 100% satisfied with my answer.

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Thank You!
Jake “The Auto Doctor”
Customer: replied 4 years ago.

So, when replacing the master cylinder... is it the standard bleeding prescedure afterwards or is there some special Lincoln procedure since it is obviously a not-standard pressure system that connects steering gear and brakes to the steering pump (Hydro boost)?

The standard bleeding procedure applies. The Hydro Boost is connected, but the systems are separate. From the master cylinder on out, it is a standard system.

The Hydro Boost is a power assist that uses pressure from the power steering pump rather than vacuum from the engine to assist the brake pressure as in a vehicle with a vacuum booster.

Let me know if you have any more questions.

Please rate my answer as OK, GOOD, or EXCELLENT. With a lower rating I will not be compensated. Please contact me with your concerns if you cannot rate my answer OK, GOOD or EXCELLENT.

Thank You!
Jake “The Auto Doctor”
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