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Old 07-23-2014, 11:41 AM
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Air getting in brakes lines after sitting

I have a 1933 ford coupe. The manual master cylinder is mounted just under the floor as a lot of these cars are. I have 11" rotors on the front and drums on the back. I use a power bleeder on the master cylinder and bleed the brakes. The brakes work great but the pedal slowly goes down over a three week period. I again bleed the brakes and there is always air in the rear lines. I have done this about 10 times. I have left the power bleeder on the master cylinder under 30 PSI for several hours and can find no leaks. This has been going on for over a year now. Several master mechanics have worked on the problem and after three new master cylinders, new cylinders on the back and two new brake lines I still have the problem. I have no idea how air is getting into the rear brake lines. I have checked all the fittings. What amazes me is I have no leakage. I have a lot of buddies scratching their heads.

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Old 07-23-2014, 11:54 AM
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Do you have a 2# residual check valve on the front line and a 10# one on the rear?
You could be drawing air in past the seals in the rear wheel cylinders.
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Old 07-23-2014, 12:02 PM
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Do you have a 2# residual check valve on the front line and a 10# one on the rear?
You could be drawing air in past the seals in the rear wheel cylinders.
Yes I do. I also have a wilwood proportioning valve on the back.
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Old 07-23-2014, 03:05 PM
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New or rebuilt or original untouched cylinders in the back?
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Old 07-24-2014, 11:54 AM
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They are new. I have put nothing but the best stuff on this car.
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Old 07-24-2014, 01:56 PM
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If air is getting into the system there is a leak somewhere. It could be a suction only leak. I have seen this before. Fluid wont leak out but air can leak in.
You said when you re-bleed the rear brakes you get air, is it on both sides or only one. If it is both sides it is most likely ahead of where the two rear wheels lines split off. A failed residual valve could be somehow sucking in air if it cant hold pressure. Pressure testing to find a vacuum type leak does not always work so your power bleeder test may not be valid. Same with A/C, generally it is easier to find an A/C leak with pressure.
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Old 07-24-2014, 03:02 PM
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Could be a bad bleeder valve too.
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Old 07-25-2014, 11:21 AM
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Could be a bad bleeder valve too.
I put new bleeder valves on the back. I hope this is not one of those problems that I will never find.
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Old 07-25-2014, 10:28 PM
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Two methods to try if all else has failed

I have had this problem on two separate cars, so this is something you might try if all else has failed.

On the first car the set up was the same as yours. I pumped up pressure on the brake pedal and wedged a 2x4 between the seat and the pedal so that there was maximum pressure on the pedal and left it there over night. This method will put much more pressure on the system than the pressure bleeder. After 24 hours get the best light you have and examine every inch of the brake system. If you have new brake fluid installed it may be hard to see if it is leaking, so put your glasses on and look close. I had the car up on jack stands. If you don't find anything get someone else to look. I found a very small leak at one of the joints in the brake line that was almost impossible to see. Once I fixed that there were no more problems with that car.

On the second car the set up was the same as yours again. We had several people go over the car and never could find a problem any where. Since the problem was on the back only, I replaced the brand new residual valve with another one and moved it up as close to the master cylinder as possible and that fixed the problem.

Hope this information is of help to you.
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Old 09-01-2014, 04:24 PM
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Quote:
Originally Posted by HotRodMan View Post
I have had this problem on two separate cars, so this is something you might try if all else has failed.

On the first car the set up was the same as yours. I pumped up pressure on the brake pedal and wedged a 2x4 between the seat and the pedal so that there was maximum pressure on the pedal and left it there over night. This method will put much more pressure on the system than the pressure bleeder. After 24 hours get the best light you have and examine every inch of the brake system. If you have new brake fluid installed it may be hard to see if it is leaking, so put your glasses on and look close. I had the car up on jack stands. If you don't find anything get someone else to look. I found a very small leak at one of the joints in the brake line that was almost impossible to see. Once I fixed that there were no more problems with that car.


In the second car the set up was the same as yours again. We had several people go over the car and never could find a problem any where. Since the problem was on the back only, I replaced the brand new residual valve with another one and moved it up as close to the master cylinder as possible and that fixed the problem.

Hope this information is of help to you.
I have tried the 2X4 on the pedal and seat and could not find anything. (Note: I was the only one looking and I don't see real well.) I'm going to put on a new wilwood master with remote reservoir and do exactly what you did. If it fixes the problem you will be my hero. I have and a bunch of my friends been working on the problem for over a year now.
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Old 09-03-2014, 04:47 AM
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Hot rod is correct in saying that the residual valve must be close to the MC. Whats happening is if the res.valve is far away the fluid is draining back, creating that air space. I have 4 wheel disc on my car and the #2 res valves are plumbed coming right out at the MC.
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