I have an 1983 XJ12, when the car is warm (after 30 mins of driving) the brake pedal looses pressure the brakes still work but the pedal travel is to the floor also I’ve noticed while this is happening coolant is overheating and spills out of the filler cap. Can these liquids ‘boil’ due to heat being generated? Perhaps a brake caliper or something? I noticed the wheels are very hot when this happens too

any help is appreciated
🙏

 

Dragging calipers can create enough heat to boil the brake fluid...which would then cause the brake pedal to go to the floor. Time for a thorough brake inspection to see what's going on.

If the brakes are really dragging then the load could overheat the engine under the right conditions....but this much drag would be probably be unmistakable from the driver's seat. More likely is the engine is overheating for other reasons

What's the history of this car?

Cheers
DD

 

Car is mint with only 45k on clock with full history. The car overheated for the last owner but the head gasket was repaired etc. The car drives beautifully and the temp gauge never rises above 90 ...... only issue is this pedal drop after 30/40 mins driving

 

Duke,

Welcome to the Forums.

Thank goodness for Google, I now know where IE is, its up there.
Please do an Intro in the New Members Area when time permits.

Since a V12, the cooling system may be inn need of some TLC, mileage etc all taken into consideration.

Have a look in the XJS Tech Section, next one down on the home page, and read my "sticky" regarding teh Cooling System. SAME system as yours, SAME issues as your car may, or will have.

The brakes I think have been well answered by Doug.

Good luck.

 

Further to Doug's points, you probably have a sticking caliper, as he indicated; but you also probably have a master cylinder going home too. If the rubbers are going - which they will be if the original cylinder is still there - this can prevent fluid returning, thus semi locking the brakes. Additionally, as the engine bay heats up, old cylinder rubbers also lose their integrity and let fluid past the plunger, thus allowing the pedal to go to the floor.
If all the brake stuff is original, and you want to use the car, I would rebuild all the calipers, replace all three flexible lines, and replace or rebuild the master cylinder. This is not an area of the car to take chances with.

 

I also had the pedal going to the floor on the XJ6 Series 3 I had. This culminated in a brown trouser moment as we went on holiday when the pedal went to the floor after descending a long hill with a sharp bend at the bottom. When we turned into a filling station for an inspection, white smoke was billowing out from the rear of the car. Once the car was up on the lift, all was revealed as a stuck rear caliper piston that had destroyed the seal, boiled the fluid and also destroyed the diff bearing seal too ! The upshot was I had to have new (actually refurbed) calipers and discs, plus diff seal; an expensive job !

The problem with inboard brakes is the road wheels dont get hot so you can miss a caliper seizure quite easily. My money is on a seized caliper somewhere and probably one or both rears as these are always neglected due to their position.

 

Quote:

The car drives beautifully and the temp gauge never rises above 90

This situation merits attention, pronto. If you have coolant boiling out but a temp gauge that never reads over 90, something is amiss. It might be a problem with the temp gauge. Or it might be localized overheating/hot spots in the cylinder heads due to lack of flow.....a problem that might not be seen on the temp gauge. This is where dropped valve seats come from.

Everything on a V12 cooling system must be 100%. Ninety-nine percent isn't good enough !

Cheers
DD

 

As we love to say, "Do it all, do it right, do it once"....and then you can forget about it for years to come.

Cheers
DD

 

That sounds Very familiar! Except in my case it was a T at the bottom of a long hill with a stop light and heavy cross traffic over in the SF Bay Area. Fortunately the light turned Green for me just at the right time, but it was an exciting few moments and quite an adrenaline rush.

I had only owned the car for 3 months when that brake booster packed it in, so I took the opportunity (as soon as I limped back to the barn) to change it and the master cylinder. 2 months later (as soon as proper parts arrived) all the calipers were replaced too.
(';')

 

Or as to coolant loss. Not over heating per the guage. But, a bad cap. does not seal....

Wrong cap. non sealer ?

CAVEAT do not drive til the brake issue is fixed.... Very bad things can happen.

 

Thanks for all the help folks!
So after inspection and tests by my mechanic turns out both rear calipers are dragging, are extremely hot after a spin (170 degrees) and both need replacing along with discs. (can the handbrake mechanism make the rear calipers bind?). The 20 psi centre cap (rad cap) is faulty too so need to sort that also. To confirm the centre cap should be a 20 PSI and it's 15 PSI for the header tank?

hopefully the head gasket is not blow a liquid head gasket tested ok, the stat tested ok too

I hear the inboard caliper removal and replacement is a nightmare of a job?

Thanks again

 

Duke, your brake overhaul will be much, much easier if you remove the entire IRS cage. Not nearly as time consuming and scary as it sounds, otherwise what you heard is correct, a nightmare of a job.

Dave

 

FYI: There is loads of stuff on the XJS section of the site about removing the IRS and doing the brakes. The XJS pre-facelift has the identical rear axle. At the same time, cage out, you can easily check the suspension pickup bracket fixing bolts tightness and the diff fixing bolt tightness; renew the diff oil, do the handbrake fix etc etc etc!
The OEM hand brake cannot overheat the discs, by the way, neither can it stop the car!

 

Duke,

Have a read of my sticky in the XJS Section regarding the cooling system, and its oddities.

You mention "stat" as in singular, tested OK, I hope that was a typo, as you have 2, and they could be the wrong length, again, its in that sticky..

The filler spout pressure cap spec means ZERO, and it is only a cap to seal the system to the outside world. As long as whatever cap is on there has a 100% perfect seal on the underside of the section you grab onto to remove/replace it, the system is happy. Down here CPC make a Blank Cap for that purpose (2001 Part Number). and many, many cars used it.

 

Removing the rear calipers isn't a nightmare of a job - it's tedious because of tight clearances. Took me about two hours to do the ones on my '73 project but I was merely rebuilding the calipers. It's better in your case to take the IRS unit out since you need to replace the discs (I've done that without removing the IRS on an XJC I had years ago but it was a PITA). While you're at it disassemble, clean, and regrease the handbrake calipers. Odds are they're stuck from disuse.

Gorgeous car, by the way.

 

Thanks metalbasher... yeah i'm wondering about the handbrake calipers and should i replace them...... do they fail like 'normal' calipers?

 

Duke
The handbrake is entirely mechanical, the problem is that it is inadequate for its purpose and the pads, in particular, are not robust enough. This link shows you what you can do to improve matters without too much difficulty:
https://www.jaguarforums.com/forum/x...7/#post2369892
start at post number 2327

Additionally, you can do the "Rolls Royce" (if you will forgive the term) full solution, as outlined in the attached.

 

Since you have cooked the rear brakes.

The diff output shaft seals are probably toast. Good time to do them while the cage is out.

The seals are OE as is the new collapsible spacer. One needed for each side.

To not do them now would be silly if in a few weeks time they leak, and that cage needs to come out just for $50 worth of parts now.

Change the diff oil while you are at it. Its been HOT also.