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Brake Fade  Rating:  Rating
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 Posted: 09-15-2005 03:33 am
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dciaccio
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Hi All:

I have my '73 JH out of rebuilding and now am in the shake down period.  I have installed an entirely new brake system including master cylinder (TR 6 ), hoses, front pads and rear shoes, and rear wheel cylinders.  I have bled the system thoroughly and have a very firm brake pedal while parked. Now the strange part. When I apply the brakes (at any speed), the pedal is firm and slows the car nicely without any pull or drag. As I come to a near stop and slightly let up on the pedal, I get a slight "rattle" feel then the pedal needs to be depressed even further to come to a complete stop. At the complete stop, I let up on the pedal (which is now almost one third to one half way down) and when re-applied (while stopped) the pedal is high and firm again.  I have checked the vacuum line to the servo and all seems ok. Any help would be appreciated.

Thanks.

Dave Ciaccio  #12248

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 Posted: 09-15-2005 12:47 pm
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Brett Gibson JH5 20497
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Sound's like they still have some air in the system, have you tried a vacumm pump to bleed them, pulling the fluid thru the bleeders is a very good way to get air trapped in the master cylinder out, plus if I remember correctly it's recommended that the master be bench bleed before installing, if you skipped that there might be your problem, but the vac pump will over come that problem.

Brett. 

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 Posted: 09-18-2005 05:19 pm
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Mark Rosenbaum
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The symptoms are characteristic of a non-sealing tipping valve inside the master cylinder.  Other possibilities in addition to the tipping valve problem include (a) air in the system, as Brett noted, and (b) an insufficient gap between the master cylinder's rear piston rod and the push rod on the front of the brake booster.

The tipping valve, which allows fluid from the reservoir to enter the business part of the cylinder, is normally forced open by the forward lip of the rear piston rod.  When the brake pedal is pressed, the piston rod moves forward and releases the valve, which is then supposed to seal.  This allows hydraulic pressure for the front brakes to develop.  Simultaneously, this developed pressure is applied to the front piston rod, which moves forward to create pressure to operate the rear brakes.

But if the tipping valve fails to seal, pressure for the front brakes never develops, and the forward piston rod does not move until the rear piston rod comes into mechanical contact with it.  Only then will the rear brake circuit develop hydraulic pressure.

The condition can be verified by removal of the brake fluid reservoir cap and examination of the reservoir contents while an assistant operates the brake pedal.  If the tipping valve is not sealing, you will see a very pronounced backflow of fluid, into the reservoir from the master cylinder. (A very small amount of backflow is normal.)

The problem is inherent in the basic design of the dual-circuit master cylinder.  A barely visible piece of debris in the brake fluid, if in the right spot, can be sufficient to keep the tipping valve from sealing.  The typical British master cylinder seems more sensitive to such debris than the typical US design, but both can suffer from it.  Replacing the master cylinder usually (not always) eliminates the problem, not because the earlier part was defective, but because the debris-containing brake fluid gets replaced with cleaner stuff.

Frequently, the problem is intermittent, with the brakes generally operating normally but failing on occasion.  This is because a piece of debris causes the tipping valve to leak, but gets flushed away as the driver repeatedly pumps the brake pedal, and the brakes then operate normally again.

The only actual cure is to use an older style single-circuit master cylinder, or, preferably, one for each brake circuit.  It will be recalled that race cars generally arrange their brakes in this manner.  Fortunately, those of us who are not driving race cars can deal with the problem by (a) being aware that the brakes may fail at any time without warning, and driving appropriately, (b) ensuring that the brake fluid is always fresh and clean, and (c) replacing it every two years (or annually, in humid environments).  Any dust or debris entering the brake fluid from an environmental source can cause the problem.  A non-silicone brake fluid will almost instantly absorb considerable moisture directly from the air, and this moisture promotes rust, which is another source of trouble-causing particles.

 

Attachment: Master cyl cross section.jpg (Downloaded 31 times)

Last edited on 09-18-2005 09:25 pm by Mark Rosenbaum

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 Posted: 07-29-2006 03:44 am
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dciaccio
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Mark & Brett:

Well its been a year later and this little problem persists. I completely bled the brake system last weekend and still have the slight brake fade after a slight "click" or "rattle" feel in the pedal at just about a stop.  The master cylinder is a rebuilt TR-6 from Apple Hydraulics and performed fine when first installed. 

Maybe a related question is the pressure differential switch.  I believe this unit has a small valve inside of it that closes off either the front or rear brake circuit in case of major leak/failure upon braking...is this true?  Could the small valve inside this unit be the culprit?

What might the next step be?

Thanks again.

Dave Ciaccio

'73 JH #12248

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 Posted: 07-29-2006 02:21 pm
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Mark Rosenbaum
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The "pressure differential switch" (actually "pressure differential warning actuator" or PDWA) doesn't shut off brake fluid in event of leaks.  All it does is to close an electrical switch that allows the dash warning light to illuminate, in the event that a gross pressure differential between the front and rear brake circuits should occur.  Even if the PDWA's interior were gutted, all that would do is create a direct connection between the front and rear brake circuits, which shouldn't cause the problem you describe.

You might try disconnecting the vacuum line to the brake booster (and capping off the manifold port), then seeing if the problem changes in some way.  If so, it may be that you have some sort of defect in the booster.  Otherwise, all I can suggest is to do a thorough visual examination of the brake mechanisms on all four wheels, in the perhaps vain hope of finding something out of sorts mechanically: incorrect assembly, wrong or loose hardware, worn pins, weak springs, sticking pistons, etc.  If this exam does not provide any clues, then most likely there's something screwy in the hydraulic part of the system, but other than a slightly leaky master cylinder piston, leaking tipping valve, I can't imagine what the problem might be.

If all else fails then perhaps it would make sense to take the car to a good brake shop and let them sort things out.

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 Posted: 08-06-2006 04:39 am
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dciaccio
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Thanks all for the help. I will do more 'bleeding' and review the hydraulics.

Will keep you posted.

Dave

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 Posted: 08-08-2006 12:34 pm
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Jon Plowe
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Hi

I had loud expensive sounding metalic tapping noise noise under the bonnet that I initially put down to imminent engine demise. Upon closer examination it was a faulty non return valve that sits on top of the servo and connects with the hose linked to the manifold. I had forgotten that I had just replaced this valve with a new one while the car was off the road for a couple of weeks. The symptoms  on the pedal were a slight rythmic pulsing with the engine running and the pedal pressed down. Changed the valve and cured problem. As Mark says servos when faulty can chuck up some strange symptoms. I don't think you will ever be thrown through the windscreen by the power of even good JH brakes!

Jon

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 Posted: 08-08-2006 05:40 pm
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John Finch
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Hey Jon, where did you get the servo non return valve? 18309 has the symptom you described and corrected by replacing the valve.

Thanks John

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 Posted: 08-09-2006 09:48 am
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Jon Plowe
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I got mine from Appleyard Engineering http://www.jensen.co.uk Very helpful for all things Jensen, however it is in the UK, although such a small part would not cost much to post.

The valve would appear a common fitment on many British cars of the 70's in the same way that callipers are often common across car types. But I could not think that there are many clubs in the US devoted to such classic British Leyland designs as the Allegro (how we used to laugh in our poo coloured, square steering wheeled cars as the back window blew out over 50mph)  However, the price Uk to US of postage would be a small price to be sure of the correct part.  I would think that Delta Motorsports would be able to source them as well

Jon

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