In the last few weeks, at times the car had been sluggish to respond to throttle. It felt as if either the tranny wasn't delivering power or that a brake somehow was "locked".

Today, while driving, this behavior happened again in a more dramatic fashion.

After driving about 5 or so miles, I pressed on the gas after stopping for traffic, just trying to get up to 30 MPH. The car would not budge. I pressed harder on the gas, and the rear tires (or at least one of them) began spinning...the car didn't move at all. I was on a slightly downhill grade, so I put it in neutral. It didn't budge. I then got out (car still in neutral) and starting pushing. Zero movement. In the past, when necessary, I've been able to push it myself on level pavement with not a lot of effort. But it wasn't going to budge a bit, even on a slight downhill grade.

I shut it off and waited about a minute. Started it...and it ran fine! Brakes fine, acceleration fine, no problems at all. I got it home (about a 5 mile drive) with no issues.

I've only had a chance to do a very brief inspect of the area around the master cylinder (which was renewed about four months ago). Nothing unusual was noted. I can't help but think that one or both front brakes where somehow "closing" themselves, but I'm not sure how to test this theory. I could be totally wrong, but I'm lacking another theory at the moment.

Thanks,

John
1987 XJ-S V12
about 70,000 miles (unless the odometer has been rolled back).

 

Sticking caliper, or rubber brake hoses collapsing inside acting like a one way valve for brake fluid. Later is more common than former, but both possible. If the hoses have never been replaced, start there.
Jon

 

You have an 1987 so you have the older vacuum servo brakes... Correct? The big tell here is the fact that you had the MC changed or rebuild recently. You need to remove the MC and adjust the piston rod. It's too long and preventing the MC piston from retracting sufficiently to allow fluid to flow back in sufficient quantity to allow the brakes to fully release quickly in some scenarios. If your piston rod adjuster nut is too rusted from brake fluid you can simply add a thick piece of grey gasket paper between the MC and booster. This will effectively shorten the rod by spacing out the MC a tiny bit.
It's possible the new or refurbished MC is in a different tolerance group than the old MC setup...And Jag made the rod adjustable for a reason. Never a bad idea to change your aged hoses too while you're at it though.

 

Thanks for that!

The question is, how much is a "tiny bit" and how do I tell when I have it right?

All the best,

John
1987 XJ-S V12

 

I use one of these





Cheers
DD

 

Since your brakes kind'a of work some times you're close. I'm NOT sure there is a specification in a service manual somewhere. But I only looked in the Sedan manual, it was at hand. In this case I often defer to and check my Porsche manual. They have a specification for everything and as such they specify 10mm of free play at the pedal. So you would feel the pedal move 10mm before engaging the piston rod against the MC. Should hear / feel a little click. I'd use the Porsche spec ONLY as a guide. On the Jag I'd shoot for 6 to 8 at the pedal and see how it felt.


No mentions, in the Jag Service manual. I guess you're just supposed to know

Well Documented in 70's Porsche Service manual. 2 pages on this subject alone. Got'a love German precision, here's an example.