The Brake Master Cylinder on my 85 XJ6 failed and I replaced it with a new one. I bench bled the MC before installation.
Now, when I take my foot off the pedal, my brake lights stay on. If I put my foot under the brake pedal and pull up, (about 1/16 -1/8 of an inch) the brake lights go off.
Prior to me replacing the brake master cylinder, I never had that problem. Why would that happen? What is the fix?
I still have to get some help to bleed the entire system just in case some air got into the system. The brake pedal is where it should be and I can lock up all 4 wheels when I test it.
Thanks

 

Maybe some slight difference in the manufacturing of the master cylinders that as altered the geometry of things .....somehow? (I'm on my first cuppa coffee)

Apply the brakes and leave the brakes light stuck on. Now slightly loosen the M/C mounting bolts. Perhaps wiggle the M/C a little. Do the brake lights turn off? If so that might be a clue.

More later when I'm clearer-headed.

Cheers
DD

 

As Doug indicates geometry may be a bit different and it may be affecting the brake light switch down near the pedal. I was having the same type of problem a while back and I bought a "Universal Brake Light Switch" off EBay. I removed my OEM brake light switch to install the new one. But... found that the new one was NC versus NO on the OEM (or the other way around, I can't remember). So, ultimately I was able to make one good switch out of the two of them. But the switch is adjustable in its holder. And that may be all you need to do.

 

Just a guess. there are two switches under the brake pedal.

1. Default is open Closes on application of the pedal Brake lights!

2. Default is closed. Application of the pedal opens the circuit. Cruise control interrupt

Caould it be that in the MC exchange, the wires were reversed/

Carl

 

Were there any instructions with the new MC? As Doug, Bill, and Carl have mentioned, slight differences in machining tolerances would change the clearance (slack) needed between the pedal and MC piston. This is created by adjusting the length of the push rod. I would guess if your lights are staying on, the new MC is allowing more clearance and the push rod is letting the brake pedal sag a little more and not reaching the cutout point on the brake switch. Shouldn't cause any operational problems, just lay on your head under the dash and adjust the switch until the lights go out, plus a few thousands.

Dave

 

press the switch behind the pedal but without pressing the pedal. and then release it to see if it remains stuck. new switch.

if by not pressing the brake pedal the switch works fine, then adjust the switch further to the front of the car to allow more clearance between the switch plunger and the back of the pedal arm.

OR. if there is no more adjustment and the pedal is not coming back all the way, install a heftier pair of springs to help pull it back. It could be the two springs at the pedal assembly pivot or the two helper springs. The two springs at the pedal assembly pivot lose their tension after years of use. They are available at SNG Barrat USA store but there is a mixup of part numbers for them. One spring is right handed, the other left handed.

 

Thanks for the input. I will try the brake switch adjustment first.
@Carl. I am also using the CC cancel switch to lock/unlock the TCC. I have a 700R4 in my XJ6.

 

YUP. That is what the CC unlock switch does in my car. CC not in service.

 

I'm sure you adjust the switch to solve the brake lites staying on. Just make sure that your brakes are not dragging due to some weirdness!

Cheers
DD

 

Doug: Funny that you mentioned that. I noticed that if I apply the brake, when I remove my foot from the pedal there is a little drag on the brake. If I use my foot to lift the pedal the car rolls a little faster. Is the push rod too long with the new MC?. I am going to get some help to bleed the brakes before I do anything else. I will also take a look at the push rod.
Thanks.

 

Dowwn the line s a thread by Jopa. MC adjustment tool. Might be applicable!!

Carl

 

Loosen the master cylinder mounting nuts a bit. See if anything changes.

Cheers
DD

 

Will do.

 

After a 9 month layoff from working on my car, I resumed work on my xj6 yesterday. As I mentioned in my initial post, I replaced the brake MC and the brake light stayed on when I released the brake pedal. The brake switch crumbled apart when I tried to adjust it. I installed an aftermarket brake switch, I added an extra spring at the brake pedal and the brake light issue has been resolved. Now a new issue has cropped up. Yesterday I had my neighbor come over to bleed the entire brake system because when I replaced the MC, I only bled the new front calipers. I did not drive the car for the last 9 months.
After bleeding all four calipers, bleeding out all of the old brake fluid from the system, I can't get the wheels to lock up.
**When I replaced the MC last December, I remember having to push the MC in to attach it to the booster. It was as if the actuating rod was a little longer with the new MC so it was starting to push the piston inward meaning the the brake stroke is now less than it was with the old MC. Before I revisit the MC installation, my question is whether the rod coming from the booster is adjustable. I will assume that when the brake pedal is at rest, the rod should not be pushing the piston inward which will cause a lesser piston stroke.
How much play should there be at the brake pedal before it starts to push the piston? I want to address this issue before I bleed the brakes again.
Thanks.

 

I don't recall. Someone here will know.

Correct.

There might be an actual spec from the manual. Lacking that I'm happy with just a fraction of an inch, maybe .025" or so. Or less. The important thing is that the m/c piston is not pushed at all when the m/c is mounted to the booster.

I bought a "master cyinder depth gauge" to check this but it might not be worth it if you don't work on a lot of cars.



Cheers
DD

 

Doug. Thank you for your prompt reply. I am going to loosen up the MC to determine how much it is pushing the piston in then determine what the next steps should be. I definitely remember that I had to push the MC against the rod coming out of the booster to catch the nuts for the MC so therein lies the problem. The question now is whether the rod is adjustable. Anyone knows? I will post results later today

 

I found this on the forum from LT1jaguar:
"Good research Joe, I have never seen that particular tool, but I understand the purpose. The video is accurate for the reason of testing and adjusting the push rod length. The instructions that came with my rebuilt booster specified a movement of 1/4" at the pedal after all normal slack in the linkage was taken up. The 1/4" movement represents the travel at the pedal (after linkage slack) before coming up against the master cylinder. As proof of this important adjustment, I separated the MC/booster more than once making adjustments to the threaded end of the push rod and when I got 5/16" travel (after linkage slack) I called it good, after all, I sez, what's the worst that can happen?! First drive and I find there is more pedal travel than I would like before the brakes apply, they work fine at that point. Now I see why directions are included with the parts."
So I would assume that the booster rod is adjustable. Hopefully LT1 will read this post and enlighten me before I take it apart.

 

I removed the brake lines from the MC and as I loosened the MC from the booster it moved out approx 1/4 inch on its own. The piston was definitely being pushed inward as I tighten the MC nuts. The booster rod is adjustable so I adjusted it until I could feel no resistance when installing the MC. I now have some play at the brake pedal. After I bleed the brakes later today (if I can find my neighbor to help) I will see how it goes from there.

 

So Sanchez, just how do you adjust the booster pushrod? Ian

 

IAN: When you remove the MC from the booster, you will see the pushrod that activates the piston. The pushrod is made up of 2 pieces. The end that goes into the MC is threaded into the larger (booster) side of the pushrod. The pushrod cannot be removed completely but you can pull it forward from the booster.
The end that goes into the MC is threaded into the larger pushrod. I held the pushrod with a pair of vice grips and used a 5/16" wrench to turn the adjustable end inwards(tighten) to shorten the rod. Turning it outwards( loosening) will make the rod longer.