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 Posted: 07-14-2019 08:42 pm
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cjwilson
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Just getting my 73 JH back on the road from being parked for years. I changed out my brake hoses all the way around. Brakes assemblies in each wheel are untouched. I put in a new or slightly used TR6 Master Cylinder and of course new fluid (The previous owner was unsure if it had ever been used and it looked unused when I received). Also my Pressure Differential Warning Light has been lit for some time now.

After about 20 minutes of great spirited driving, pedal went soft. It came back up after a pump or two. I got the car home with no issues other than having to pump brakes once or twice each time in order to stop. Popped the hood and see the brake fluid levels are good, but maybe dropped slightly, maybe not. But it was startling to see the fluid in the rear chamber (front brakes) to be dark brown. Front part of reservoir (rear brakes) fluid was still new looking.

Today I pulled the pressure differential switch, thinking an o-ring there has failed, causing both the dark fluid and the dash light. O-rings looked ok, no pitting in the cylinder and was stiff to move pistons from side to side. Also, there has been a bit of brake fluid showing around the switch for a while. I unplugged the switch and the dash light stayed lit.

I would sure appreciate any feedback here saying where to start and if anyone has had this same issue!

Attachment: IMG_0666.jpg (Downloaded 62 times)

Last edited on 07-14-2019 08:46 pm by cjwilson

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 Posted: 07-14-2019 10:56 pm
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redracer
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cjw: here's a post I did a few years ago(use the "SEARCH" box at top). Read some of the other comments as well
Hope this helps:
Brakes > Pressure Differential Warning Actuator (8 replies - 4015 views) 08-29-2017
Dealing with a North American Safety device, the Pressure Differential Warning Actuator. Just finished the car and the brake fail light stays on ...

posted: 08-30-2017 10:04 am
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redracer
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I wrote about this over 25 years ago and had it published in the White Lady, our now defunct mag. for all Jensen owners.I believe I had it entitled "Another one of Nadir's stupid safety ideas"(or something to that effect). Our dual master cylinders had one side do front brakes and the other the rears, unlike modern cars which do diagonal pressure, i.e. left front with right rear, etc.. You can live and drive easily without the rears(I have), but not the fronts, and you will NOT need a light to tell you so. Plus, bleeding the brakes has sometimes "locked" the pins on one side or the other causing the light to always be on.
The "fix" is fairly simple and requires you(or friend) to have the small 1/8" pipe tap(27 threads/inch and NOT the British pipe of 28 threads/inch, which is what the oil pressure line fittings are). Remove both ends of the switch, remove the 2 sliding plugs and the small steel ball, then turn the tap 12-13 complete turns in each end(no drilling is necessary as the holes are the correct size to begin with!!). Using 2 brass allen plugs(takes a 3/16" allen wrench), wrap a little teflon tape on the "starting" ends of each plug and install, being sure none of the teflon is exposed in the chambers(snip extra off).
(if you can't find the brass plugs, I still have plenty)

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 Posted: 07-14-2019 11:24 pm
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cjwilson
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Thanks Red Racer. Yes, I have read your post several times. I've already been considering the change to the actuator. I looked through my taps and don't have 1/8 27 threads, so will have to run to town and purchase.

More concerned with what has failed to cause the discolor and loss of pedal.

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 Posted: 07-15-2019 04:45 am
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Tom Bradley
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This looks like what happened after I bought a rebuild kit for my master cylinder. When I took it apart I found that the O-rings were already going bad. Apparently someone put Buna-N O-rings in the kit, which do not hold up with brake fluid instead of Viton O-rings. My guess is that if you take the master cylinder apart you will find one or more o-rings disintegrating.

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 Posted: 07-15-2019 10:02 am
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cjwilson
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OK. Thanks. Will that cause loss of pedal too?

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 Posted: 07-15-2019 07:11 pm
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Tom Bradley
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Yes, having to pump the brake pedal to maintain brake pressure was the first thing I noticed after I had been driving with the rebuilt master cylinder for awhile. Then, when I looked, the fluid was also darker. When I pulled it back off and took it back apart is when I saw the bad o-rings.

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 Posted: 07-15-2019 08:00 pm
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cjwilson
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Gotcha Thanks Tom!!

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 Posted: 07-16-2019 11:14 am
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cjwilson
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So, I've got a new rebuild kit on the way from Delta. But while I'm waiting, I need to address the light on the dash. Like I mentioned, it stays lit even if the switch is unplugged. So either the switch is making constant contact internally, of something else is failed. I'll pull out my schematic and start hunting.

I think I read here that the switch for the parking brake is impossible to find in the car. If anyone cared to share where it is to save me the trouble, I'd sure appreciate it!

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 Posted: 07-17-2019 12:50 pm
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Brett Gibson JH5 20497
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Parking brake switch is under the car directly at the hand brake lever, it's a simple switch that most times just goes out of adjustment, sounds like yours is engaged, can't remember if you have to pull the drive shaft to get at it, but it's not a difficult job to adjust.

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 Posted: 07-17-2019 03:07 pm
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cjwilson
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Great. Thanks!!

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