 Reset Master Cylinder Warning Piston? |
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| bkrantz |
 Mar 30 2022, 08:46 PM
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#1
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914 Guru  Group: Members Posts: 7,762 Joined: 3-August 19 From: SW Colorado Member No.: 23,343 Region Association: Rocky Mountains 
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Seems like the small diameter piston that triggers the brake fluid pressure warning light in my new MC is not centered, so the small diameter segment is not aligned with the hole for the switch plunger. This photo shows the piston removed from my old MC.
Does anyone know how to reset or recenter the piston in the MC? (not reset the plunger on the switch) |
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| bkrantz |
Mar 31 2022, 03:53 PM
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#2
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914 Guru Group: Members Posts: 7,762 Joined: 3-August 19 From: SW Colorado Member No.: 23,343 Region Association: Rocky Mountains
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| PatMc |
Mar 31 2022, 05:41 PM
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#3
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Member Group: Members Posts: 120 Joined: 27-June 21 From: Long Beach Member No.: 25,669 Region Association: None
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QUOTE(bkrantz @ Mar 30 2022, 09:46 PM)  Seems like the small diameter piston that triggers the brake fluid pressure warning light in my new MC is not centered, so the small diameter segment is not aligned with the hole for the switch plunger. This photo shows the piston removed from my old MC. Does anyone know how to reset or recenter the piston in the MC? (not reset the plunger on the switch) If you can tell which way it moved...have someone step on the pedal and crack the line opposite the way it moved...that should move it back. You might also get in there with the pick after removing the switch and recenter it...if you're good |
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| bkrantz |
Apr 1 2022, 07:27 PM
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#4
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914 Guru Group: Members Posts: 7,762 Joined: 3-August 19 From: SW Colorado Member No.: 23,343 Region Association: Rocky Mountains
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Thanks. I tried prying, and then pressing on the pedal with first the front and then the rear circuit open. Neither approach moved the warning valve piston.
I did confirm that the front and rear brakes do work when the opposite circuit is open, even though the pedal bottoms out. |
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| GregAmy |
Apr 1 2022, 08:14 PM
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#5
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Advanced Member Group: Members Posts: 2,298 Joined: 22-February 13 From: Middletown CT Member No.: 15,565 Region Association: North East States
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There's springs on either side to center it...if it's moving smoothly in the bore.
Sounds like you need to rebuild/replace the master. |
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| bkrantz |
Apr 2 2022, 09:57 PM
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#6
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914 Guru Group: Members Posts: 7,762 Joined: 3-August 19 From: SW Colorado Member No.: 23,343 Region Association: Rocky Mountains
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See my build thread for more details, but I ended up removing the MC from the car, and then removing the warning system piston from the MC.
The first problem is that the centering spring is missing (compare to the piston from my old MC on top). Remember, this is a brand new MC. Attached thumbnail(s) 
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| bkrantz |
Apr 2 2022, 10:01 PM
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#7
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914 Guru Group: Members Posts: 7,762 Joined: 3-August 19 From: SW Colorado Member No.: 23,343 Region Association: Rocky Mountains
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The second problem is that the distance from the internal bore to the warning switch mounting face is about .04" less on the new one. This means that even with the piston centered, the pumper on the warning switch is depressed enough to close the switch and turn on the warning light.
Not happy with this bargain MC. Attached thumbnail(s) 
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| robkammer |
Apr 3 2022, 12:34 PM
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#8
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Member Group: Members Posts: 179 Joined: 7-January 21 From: Vermilion Ohio Member No.: 25,049 Region Association: North East States |
Can I could jump in the middle here with a question? My new MC of 1 year has been leaking through the switch and a replacement was sent. Waiting on a new washer. I just noticed that there is a small rubber cap on the plunger of the new one. Looked at the old one and it had one as well. Should I have removed this before putting the boot on?
And with a plastic plunger and only one wire at the switch, how does it complete the circuit? There is a lot of black tape on the wire at the boot so I may yet another old hack to deal with. Rob |
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| GregAmy |
Apr 3 2022, 02:08 PM
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#9
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Advanced Member Group: Members Posts: 2,298 Joined: 22-February 13 From: Middletown CT Member No.: 15,565 Region Association: North East States
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QUOTE(bkrantz @ Apr 2 2022, 11:57 PM) Remember, this is a brand new MC. Not good. QUOTE(robkammer @ Apr 3 2022, 02:34 PM) My new MC of 1 year has been leaking through the switch... There should be no fluid in that center chamber where the switch is. Look at the photo above; see how there's a seal on each side of it? Fluid in that center chamber indicates a leaking seal. I had that same exact problem with a "new" master cylinder last Fall. Vendor sent me a replacement. Might be time for us to identify a good quality MC repair kit and rebuilt known-OE ones. Greg |
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| robkammer |
Apr 3 2022, 05:02 PM
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#10
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Member Group: Members Posts: 179 Joined: 7-January 21 From: Vermilion Ohio Member No.: 25,049 Region Association: North East States |
QUOTE(GregAmy @ Apr 3 2022, 04:08 PM) QUOTE(bkrantz @ Apr 2 2022, 11:57 PM) Remember, this is a brand new MC. Not good. QUOTE(robkammer @ Apr 3 2022, 02:34 PM) My new MC of 1 year has been leaking through the switch... There should be no fluid in that center chamber where the switch is. Look at the photo above; see how there's a seal on each side of it? Fluid in that center chamber indicates a leaking seal. I had that same exact problem with a "new" master cylinder last Fall. Vendor sent me a replacement. Might be time for us to identify a good quality MC repair kit and rebuilt known-OE ones. Greg Thanks Greg. I think. So if I understand correctly, the new 914 rubber ez install MC is leaking where it should not? I wish I would have saved the old mc or at least pulled it apart so I had a better feel for how it's put together. When I talked to Mark at 914R about the switch he didn't hesitate to send a new one, but if what you're saying is true then I'm stuck with the chinese EZ MC and that could be the source of my less than ideal braking. Huh. |
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| bkrantz |
Apr 3 2022, 06:58 PM
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#11
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914 Guru Group: Members Posts: 7,762 Joined: 3-August 19 From: SW Colorado Member No.: 23,343 Region Association: Rocky Mountains
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That's right. There should be no brake fluid in the port that the switch plunger sticks into. The seals on the warning piston should prevent that.
And the switch itself should have an O-ring. The single lead switch grounds the brown/white wire through the body of the MC to the chassis. Hopefully. |
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| robkammer |
Apr 5 2022, 09:18 AM
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#12
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Member Group: Members Posts: 179 Joined: 7-January 21 From: Vermilion Ohio Member No.: 25,049 Region Association: North East States |
Mark called me back yesterday and feels it may be a problem with the O-ring. I'm kicking myself for tossing the old one, it was probably an ATE, but with all the chatter about upgrading to 19mm and having a brand new one in hand, out it went.
I'l report back in a week or so when the parts get here |
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