Brake Failure

[jamesmdavis]

Good Day From Houston:
Yesterday I got the TC all shined up for a car show. I started my final check ride, put the brakes on and the pedal promptly went to the floor!!! Amazing how fast one's thoughts get focused!! Thanks to the hand brake I was able to get the old girl back to the garage. I checked the level in the MC. It was just above the bridge. No sign of any leaks anywhere. The previous day I had tested the brakes HARD and they performed flawlessly. I intend to pull drums and inspect, and also bleed them. What other ideas does this august body have?? Thanks.

Jim Davis
TC 7225

[Steve Simmons]

My first thought would be a master cylinder problem. Maybe failed rubber bits?

[Duncan M]

My first thought the same as Steve on m/c. I would be interested in how the pedal returns from the floor, it it does. Also, the TC brakes are sensitive to air in the lines; it seems to get in there over time, somehow.

[jamesmdavis]

Duncan - the pedal returns to the initial position like a rocket....Jim

[Duncan M]

If the master cylinder piston rod is returning fast with the pedal, you might just need to bleed.

[frenchblatter]

I wouldn't look any further. Get a repair kit (from whoever supplied the m/c) and fit new seals.

As I've said on an email, it can't have air in the system if the cylinder is still full. For air to get in, fluid has to come out.

[Duncan M]

Not much history given.

[jamesmdavis]

Thanks to all of you for your ideas. I checked the brake lines,MC, etc with no indication of leakage. Then I bled the line out. Much air in there. The brakes came back after bleeding. Perhaps when I tested the brakes HARD the day before, I explosed some intermittent crack etc in the system which only shows up with hard use. I plan to give it a rigorous test after my diff comes back from being worked over. I'm on pins and needles driving it now and will try to break the fault during the next hard test. I'll keep the group informed. Thanks again.

Jim Davis
TC 7225
Houston

[Steve Simmons]

Great news. Could be that there was some trapped air that got pushed through. Assuming all of your hoses, bleeders, flare nuts, etc are very clean and dry, you should be able to detect any leaks after a hard run. Good luck!

[SteveW]

Have you looked for leaks at the piston seals in the slave cylinders on each wheel. As TC's don't have dual circuit brakes like modern cars, the failure of one would cause loss of pressure in them all. Maybe your hard braking unseated one and its now resealed again. When you bled the brakes, was there a particular location where the air came out the most?

Also (and this might just be ill informed speculation), if a slave cylinder piston temporarily jammed in a mostly open position and you then lifted your foot off the brake pedal, could you have sucked air in around the rubber seals?

[Duncan M]

A proper bleeding will tell you a lot about it.

[frenchblatter]

Duncan, your brakes are a sealed system. You fill it with fluid and bleed it to expel all the air. Therefore, for air to sudenly be in the system, fluid must come out. The OP said the master cylinder was full the day before so where did the fluid go.

[Steve Simmons]

A very small amount of air will have a huge effect on the brakes. You may not notice the fluid level change at all. We're talking about a drinking straw with of air rendering the brakes inoperable. Air can also be pulled in through the wheel cylinder seals in some situations, especially if the brake shoe return springs are weak or missing.

[Richard Michell]

Norm, for air to leak in fluid does not have to leak out IMHO. The fluid level can just rise. As Steve says, it does not need much air to render the pedal soft.

Jim, you said that the pedal returned like a rocket. This may be a contributor to your problem. If you did a very hard application in a car that does not get a lot of use, the pistons in the wheel cylinders may not have been able to retract as fast as the one in the master cylinder, thus creating a partial vacuum in one or more wheel cylinders.

The wheel cylinder pistons are pushed back by the shoes/shoe springs and the master cylinder piston by the spring in the master cylinder. As Steve says, weak shoe return springs (or rusty shoe pivot points) can cause this mismatch.

When you bled, was the air at the wheel cylinders - came out early in the bleed process - or at the master cylinder - took some fluid to pass before the air emerged? Was it at all wheels?

[jamesmdavis]

There was quite a bit of air in the furthest wheel. Four pumps before brake fluid began to appear. I've never had that experience before when I doing yearly bleeding. The others had minimal air. I had previously changed out all the springs at the wheels and pedal. I suspect the furthest cylinder has a small crack etc that was activated by the hard testing. I will replace that cylinder, etc. to be on the safe side.

Jim

[stephen stierman]

Suggest you peel back the rubber boots at each end of the cylinder to see if there is any seepage at that point. Those cylinders are fairly stout material and I might suspect a leaking rubber cup rather than a failure of any other sort.

[Duncan M]

It is only sealed in a sense, as Norman describes. Moisture, for instance, does finds its way into the brake fluid. Every time the brakes are applied little tiny bubbles in the fluid are formed, and like water in fluid can get distributed throughout the system. Air happens.

Silicone fluid is prone to all sorts of problems.

[stephen stierman]

Duncan, I would have to disagree with that last statement. I have not touched my Morgan's hydraulic system, rebuilt and filled with silicone fluid in about twenty years, I know, I know it should have the brake fluid flushed by now. In any case it works just fine and exhibits no bad behavior at all.

My TC, even though I went through everything and installed new cylinders would eventually require being bled again after a soft peddle would eventually show up after use. I found a couple of cylinders that seemed to show leakage when I peeled back the boots. After replacing cylinders all is well, no rebleeding required. If you have to bleed occasionally you have a leak in the system.

[frenchblatter]

The point is that if fluid leaks out air will be sucked in. The return of the master cylinder piston creates a vacuum in the system which will suck air from outside if theres a place for it to get in.

My experiance is that a system will still work with quite a bit of air in it. I have had cars with soft pedals, almost to the floor but the brakes works. When bleeding a lot of air came out from all four bleed nipples.

Even a system where the pedal goes to the floor can normally be pumped up so the brakes work.

[stephen stierman]

Norman, no disagreement here, what you describe is pretty typical of a system with a leak somewhere and my goal would be to fix it. A TC is no different than any other system and when functioning properly the peddle should be hard and not require much movement to actuate the brakes.